mkxp.pro: Shorten keyword lenght to choose binding

'BINDING=BINDING_MRI' => 'BINDING=MRI'

Also error out if multiple bindings are chosen.

README.md

@@ -50,11 +50,11 @@ qmake will use pkg-config to locate the respective include/library paths. If you
 
 The exception is boost, which is weird in that it still hasn't managed to pull off pkg-config support (seriously?). *If you installed boost in a non-standard prefix*, you will need to pass its include path via `BOOST_I` and library path via `BOOST_L`, either as direct arguments to qmake (`qmake BOOST_I="/usr/include" ...`) or via environment variables.
 
-**MRI-Binding**: pkg-config will look for `ruby-2.1.pc`, but you can modify mkxp.pro to use 2.0 instead. This is the default binding, so no arguments to qmake needed (`BINDING=BINDING_MRI` to be explicit).
+**MRI-Binding**: pkg-config will look for `ruby-2.1.pc`, but you can modify mkxp.pro to use 2.0 instead. This is the default binding, so no arguments to qmake needed (`BINDING=MRI` to be explicit).
 
-**MRuby-Binding**: place the "mruby" folder into the project folder and build it first. Add `BINDING=BINDING_MRUBY` to qmake's arguments.
+**MRuby-Binding**: place the "mruby" folder into the project folder and build it first. Add `BINDING=MRUBY` to qmake's arguments.
 
-**Null-Binding**: Add `BINDING=BINDING_NULL` to qmake's arguments.
+**Null-Binding**: Add `BINDING=NULL` to qmake's arguments.
 
 ### Supported image/audio formats
 These depend on the SDL auxiliary libraries. For maximum RGSS compliance, build SDL2_image with png/jpg support, and SDL_sound with oggvorbis/wav/mp3 support.

mkxp.pro

@@ -7,6 +7,52 @@ DEPENDPATH += src shader assets
 INCLUDEPATH += . src
 LIBS += -lGL
 
+CONFIG(release, debug|release): DEFINES += NDEBUG
+
+isEmpty(BINDING) {
+	BINDING = MRI
+}
+
+contains(BINDING, MRI) {
+	contains(_HAVE_BINDING, YES) {
+		error("Only one binding may be selected")
+	}
+	_HAVE_BINDING = YES
+
+	CONFIG += BINDING_MRI
+}
+
+contains(BINDING, MRUBY) {
+	contains(_HAVE_BINDING, YES) {
+		error("Only one binding may be selected")
+	}
+	_HAVE_BINDING = YES
+
+	CONFIG += BINDING_MRUBY
+}
+
+contains(BINDING, NULL) {
+	contains(_HAVE_BINDING, YES) {
+		error("Only one binding may be selected")
+	}
+	_HAVE_BINDING = YES
+
+	CONFIG += BINDING_NULL
+}
+
+RGSS2 {
+	DEFINES += RGSS2
+}
+
+unix {
+	CONFIG += link_pkgconfig
+	PKGCONFIG += sigc++-2.0 glew pixman-1 zlib physfs \
+	             sdl2 SDL2_image SDL2_ttf SDL_sound openal
+
+	RGSS2 {
+		PKGCONFIG += vorbisfile
+	}
+
 	# Deal with boost paths...
 	isEmpty(BOOST_I) {
 		BOOST_I = $$(BOOST_I)
@@ -25,28 +71,6 @@ else {
 	}
 
 	LIBS += -lboost_program_options
-
-
-CONFIG(release, debug|release): DEFINES += NDEBUG
-
-isEmpty(BINDING) {
-	BINDING = BINDING_MRI
-}
-
-CONFIG += $$BINDING
-
-RGSS2 {
-	DEFINES += RGSS2
-}
-
-unix {
-	CONFIG += link_pkgconfig
-	PKGCONFIG += sigc++-2.0 glew pixman-1 zlib physfs \
-	             sdl2 SDL2_image SDL2_ttf SDL_sound openal
-
-	RGSS2 {
-		PKGCONFIG += vorbisfile
-	}
 }
 
 # Input