BUILD: Unify how _posix is computed

However, the current approach of determining _posix based on _host_os is
flawed and should be replaced by feature detection; added a TODO about
this.

configure

@@ -1548,7 +1548,6 @@ case $_host_os in
 		LDFLAGS="$LDFLAGS --sysroot=$ANDROID_NDK/platforms/android-4/arch-arm"
 		LDFLAGS="$LDFLAGS -mthumb-interwork"
 		add_line_to_config_mk "ANDROID_SDK = $ANDROID_SDK"
-		_posix=yes
 		_seq_midi=no
 		;;
 	beos*)
@@ -1558,12 +1557,8 @@ case $_host_os in
 		CFLAGS="-I/boot/home/config/include"
 		CXXFLAGS="$CXXFLAGS -fhuge-objects"
 		LIBS="$LIBS -lbind -lsocket"
-		_posix=yes
 		_seq_midi=no
 		;;
-	bsd* | hpux* | netbsd* | openbsd* | sunos*)
-		_posix=yes
-		;;
 	cygwin*)
 		echo ERROR: Cygwin building is not supported by ScummVM anymore. Consider using MinGW.
 		exit 1
@@ -1572,7 +1567,6 @@ case $_host_os in
 		DEFINES="$DEFINES -DMACOSX"
 		LIBS="$LIBS -framework AudioUnit -framework AudioToolbox -framework Carbon -framework CoreMIDI"
 		add_line_to_config_mk 'MACOSX = 1'
-		_posix=yes
 		;;
 	dreamcast)
 		DEFINES="$DEFINES -D__DC__ -DNONSTANDARD_PORT"
@@ -1598,7 +1592,6 @@ case $_host_os in
 	freebsd*)
 		LDFLAGS="$LDFLAGS -L/usr/local/lib"
 		CXXFLAGS="$CXXFLAGS -I/usr/local/include"
-		_posix=yes
 		;;
 	gamecube)
 		CXXFLAGS="$CXXFLAGS -Os -mogc -mcpu=750 -meabi -mhard-float"
@@ -1616,14 +1609,12 @@ case $_host_os in
 		DEFINES="$DEFINES -DSYSTEM_NOT_SUPPORTING_D_TYPE"
 		# Needs -lnetwork for the timidity MIDI driver
 		LIBS="$LIBS -lnetwork"
-		_posix=yes
 		_seq_midi=no
 		;;
 	irix*)
 		DEFINES="$DEFINES -DIRIX -DSYSTEM_NOT_SUPPORTING_D_TYPE"
 		LIBS="$LIBS -lmd -lfastm -lm"
 		_ranlib=:
-		_posix=yes
 		;;
 	linux* | uclinux*)
 		# When not cross-compiling, enable large file support, but don't
@@ -1631,7 +1622,6 @@ case $_host_os in
 		if test -z "$_host"; then
 			CXXFLAGS="$CXXFLAGS $(getconf LFS_CFLAGS 2>/dev/null)"
 		fi
-		_posix=yes
 		DEFINES="$DEFINES -DLUA_USE_POSIX"
 		;;
 	mingw*)
@@ -1642,7 +1632,6 @@ case $_host_os in
 		;;
 	mint*)
 		DEFINES="$DEFINES -DSYSTEM_NOT_SUPPORTING_D_TYPE"
-		_posix=yes
 		;;
 	n64)
 		DEFINES="$DEFINES -D__N64__ -DLIMIT_FPS -DNONSTANDARD_PORT"
@@ -1650,9 +1639,6 @@ case $_host_os in
 		DEFINES="$DEFINES -DDISABLE_FANCY_THEMES -DDISABLE_DOSBOX_OPL -DDISABLE_SID -DDISABLE_NES_APU"
 		DEFINES="$DEFINES -DREDUCE_MEMORY_USAGE"
 		;;
-	os2-emx*)
-		_posix=yes	# FIXME??? Why??
-		;;
 	ps2)
 		# TODO ps2
 		CXXFLAGS="$CXXFLAGS -G2"
@@ -1669,7 +1655,6 @@ case $_host_os in
 		DEFINES="$DEFINES -DSOLARIS -DSYSTEM_NOT_SUPPORTING_D_TYPE"
 		# Needs -lbind -lsocket for the timidity MIDI driver
 		LIBS="$LIBS -lnsl -lsocket"
-		_posix=yes
 		;;
 	webos)
 		CXXFLAGS="$CXXFLAGS -I$WEBOS_PDK/include -I$WEBOS_PDK/include/SDL -I$WEBOS_PDK/device/usr/include"
@@ -1679,7 +1664,6 @@ case $_host_os in
 		LDFLAGS="$LDFLAGS -Wl,--allow-shlib-undefined"
 		LDFLAGS="$LDFLAGS --sysroot=$WEBOS_PDK/arm-gcc/sysroot"
 		add_line_to_config_mk "WEBOS_SDK = $WEBOS_SDK"
-		_posix=yes
 		_seq_midi=no
 		;;
 	wii)
@@ -1699,19 +1683,42 @@ case $_host_os in
 		DEFINES="$DEFINES -D__ARM__ -D_ARM_ -DUNICODE -DFPM_DEFAULT -DNONSTANDARD_PORT"
 		DEFINES="$DEFINES -DWIN32 -Dcdecl= -D__cdecl__="
 		;;
-	# given this is a shell script assume some type of posix
+esac
-	*)
+
-		echo "WARNING: could not establish system type, assuming unix like"
+#
+# Determine whether host is POSIX compliant, or at least POSIX
+# compatible enough to support our POSIX code (including dlsym(),
+# mkdir() and some other APIs).
+#
+# TODO: Instead of basing this on the host name, we should really base
+# this on the presence of features (such as the dlsym and mkdir APIs).
+#
+echo_n "Checking if host is POSIX compliant... "
+case $_host_os in
+	amigaos* | cygwin* | dreamcast | ds | gamecube | mingw* | n64 | ps2 | psp | wii | wince)
+		;;
+	android | beos* | bsd* | darwin* | freebsd* | gph-linux | haiku* | hpux* | iphone | irix* | linux* | mint* | netbsd* | openbsd* | solaris* | sunos* | uclinux* | webos)
 		_posix=yes
 		;;
+	os2-emx*)
+		_posix=yes	# FIXME: Really???
+		;;
+	*)
+		# given this is a shell script, we might assume some type of posix.
+		# However, the host system might be a totally different one, so
+		# we can assume nothing about it.
+		# Indeed, as mentioned further above, we really should test for the
+		# presences of relevant APIs on the host anyway...
+		_posix=no
+		;;
 esac
+echo $_posix
 
 if test -n "$_host"; then
 	# Cross-compiling mode - add your target here if needed
 	echo "Cross-compiling to $_host"
 	case "$_host" in
 		android | android-v7a)
-			_posix=yes
 			# we link a .so as default
 			LDFLAGS="$LDFLAGS -shared -Wl,-Bsymbolic,--no-undefined"
 			HOSTEXEPRE=lib
@@ -1724,11 +1731,9 @@ if test -n "$_host"; then
 			_timidity=no
 			;;
 		arm-linux|arm*-linux-gnueabi|arm-*-linux)
-			_posix=yes
 			;;
 		arm-riscos|linupy)
 			DEFINES="$DEFINES -DLINUPY"
-			_posix=yes
 			;;
 		bfin*)
 			;;
@@ -1744,7 +1749,6 @@ if test -n "$_host"; then
 			fi
 			CXXFLAGS="$CXXFLAGS -mcpu=arm926ej-s -mtune=arm926ej-s"
 			ASFLAGS="$ASFLAGS"
-			_posix=yes
 			_backend="gph"
 			_build_hq_scalers=no
 			_vkeybd=yes
@@ -1818,7 +1822,6 @@ if test -n "$_host"; then
 			CXXFLAGS="$CXXFLAGS -march=armv4t"
 			ASFLAGS="$ASFLAGS -mfloat-abi=soft"
 			LDFLAGS="$LDFLAGS -static"
-			_posix=yes
 			_backend="gph"
 			_build_hq_scalers=no
 			_vkeybd=yes
@@ -1836,7 +1839,6 @@ if test -n "$_host"; then
 			fi
 			CXXFLAGS="$CXXFLAGS -mcpu=arm926ej-s -mtune=arm926ej-s"
 			ASFLAGS="$ASFLAGS -mfloat-abi=soft"
-			_posix=yes
 			_backend="gph"
 			_build_hq_scalers=no
 			_vkeybd=yes
@@ -1847,14 +1849,12 @@ if test -n "$_host"; then
 			;;
 		iphone)
 			DEFINES="$DEFINES -DIPHONE"
-			_posix=yes
 			_backend="iphone"
 			_build_hq_scalers=no
 			_seq_midi=no
 			;;
 		m68k-atari-mint)
 			DEFINES="$DEFINES -DSYSTEM_NOT_SUPPORTING_D_TYPE"
-			_posix=yes
 			_ranlib=m68k-atari-mint-ranlib
 			_ar="m68k-atari-mint-ar cru"
 			_seq_midi=no
@@ -1872,7 +1872,6 @@ if test -n "$_host"; then
 		motoezx)
 			DEFINES="$DEFINES -DMOTOEZX"
 			ASFLAGS="$ASFLAGS -mfpu=vfp"
-			_posix=yes
 			_backend="linuxmoto"
 			_build_hq_scalers=no
 			_mt32emu=no
@@ -1883,7 +1882,6 @@ if test -n "$_host"; then
 		motomagx)
 			DEFINES="$DEFINES -DMOTOMAGX"
 			ASFLAGS="$ASFLAGS -mfpu=vfp"
-			_posix=yes
 			_backend="linuxmoto"
 			_build_hq_scalers=no
 			_mt32emu=no
@@ -1915,7 +1913,6 @@ if test -n "$_host"; then
 			;;
 		neuros)
 			DEFINES="$DEFINES -DNEUROS"
-			_posix=yes
 			_backend='null'
 			_build_hq_scalers=no
 			_mt32emu=no
@@ -1930,7 +1927,6 @@ if test -n "$_host"; then
 			fi
 			CXXFLAGS="$CXXFLAGS -march=armv7-a -mtune=cortex-a8 -mfpu=neon"
 			ASFLAGS="$ASFLAGS -mfloat-abi=soft"
-			_posix=yes
 			_backend="openpandora"
 			_build_hq_scalers=yes
 			_vkeybd=no
@@ -1979,13 +1975,11 @@ if test -n "$_host"; then
 			DEFINES="$DEFINES -DSAMSUNGTV -DDISABLE_COMMAND_LINE"
 			ASFLAGS="$ASFLAGS -mfpu=vfp"
 			HOSTEXEEXT=".so"
-			_posix=yes
 			_backend="samsungtv"
 			_mt32emu=no
 			_vkeybd=yes
 			;;
 		webos)
-			_posix=yes
 			_backend="webos"
 			_port_mk="backends/platform/webos/webos.mk"
 			_build_scalers=no