CONFIGURE: Add a static builds switch

Configure will still build every target with shared objects by default.

Done:
- Added a configure switch (--enable-static) for static builds
- Right now only tested for SDL targets (checked working on AmigaOS4 with SDL1/2)

Missing:
- Add the platforms that need the switch for cross-compiling (macOS, iOS, probably more)
(I can´t do that myself as i have never cross-compiled and wouldn´t be able to check)
 
@csnover asked on the forums to create a PR for it, so, with my limited (non-existant is more like it) knowledge, i wrapped something up.
I *know* that it isn´t enough, as there already is some kind of static builds flag in configure (_engines_built_static). As i don´treally know if this is the same or a different cause (static plugins?), i´m not touching it.

But please feel free to add or change what needs to be done.

configure

@@ -957,6 +957,7 @@ Game engines:
     The values of <engine name> for these options are as follows:
 $engines_help
 Optional Features:
+  --enable-static          build a static binary instead of using shared objects
   --enable-c++11           build as C++11 if the compiler allows that
   --disable-debug          disable building with debugging symbols
   --enable-Werror          treat warnings as errors
@@ -1100,6 +1101,7 @@ EOF
 
 for ac_option in $@; do
 	case "$ac_option" in
+	--enable-static)         _static_build=yes    ;;
 	--disable-16bit)          _16bit=no       ;;
 	--enable-highres)         _highres=yes    ;;
 	--disable-highres)        _highres=no     ;;
@@ -3481,7 +3483,11 @@ case $_backend in
 	androidsdl | dingux | gph | linuxmoto | maemo | openpandora | samsungtv | sdl)
 		find_sdlconfig
 		append_var INCLUDES "`$_sdlconfig --prefix="$_sdlpath" --cflags`"
+		if test "$_static_build" = yes ; then
+			append_var LIBS "`$_sdlconfig --prefix="$_sdlpath" --static-libs`"
+		else
 			append_var LIBS "`$_sdlconfig --prefix="$_sdlpath" --libs`"
+		fi
 		append_var DEFINES "-DSDL_BACKEND"
 		add_line_to_config_mk "SDL_BACKEND = 1"