BUILD: Fix broken Dreamcast ronin handling & feature detection

$(ronindir) and the explicit crt0 may have happened to work because
library detection state for libmad was forced, but it was always
breaking the feature detection of the build system. Now we can
compile to Dreamcast using the normal detection system.

backends/platform/dc/dreamcast.mk

@@ -1,6 +1,4 @@
 
-ronindir = /usr/local/ronin
-
 CC := $(CXX)
 ASFLAGS := $(CXXFLAGS)
 

configure

@@ -1774,6 +1774,12 @@ android)
 		exit 1
 	fi
 	;;
+dreamcast)
+	if test -z "$RONINDIR"; then
+		echo "Please set RONINDIR in your environment. export RONINDIR=<path to libronin>"
+		exit 1
+	fi
+	;;
 n64)
 	if test -z "$N64SDK"; then
 		echo "Please set N64SDK in your environment. export N64SDK=<path to n64 sdk>"
@@ -3032,13 +3038,6 @@ if test -n "$_host"; then
 			append_var CXXFLAGS "-fdelete-null-pointer-checks"
 			_backend="dc"
 			_build_scalers=no
-			_mad=yes
-			_zlib=yes
-			if test -z "$RONINDIR"; then
-				add_line_to_config_mk "ronindir := /usr/local/ronin"
-			else
-				add_line_to_config_mk "ronindir := $RONINDIR"
-			fi
 			_port_mk="backends/platform/dc/dreamcast.mk"
 			;;
 		ds)
@@ -3409,11 +3408,9 @@ case $_backend in
 		;;
 	dc)
 		append_var INCLUDES '-I$(srcdir)/backends/platform/dc'
-		append_var INCLUDES '-isystem $(ronindir)/include'
 		append_var LDFLAGS "-Wl,-Ttext,0x8c010000"
 		append_var LDFLAGS "-nostartfiles"
-		append_var LDFLAGS '$(ronindir)/lib/crt0.o'
-		append_var LDFLAGS '-L$(ronindir)/lib'
+		append_var LIBS "$RONINDIR/lib/crt0.o"
 		# Enable serial debugging output only when --enable-debug is passed
 		if test "$_release_build" = yes -o "$_debug_build" != yes; then
 			append_var LIBS "-lronin-noserial -lm"