Use the "target"/"targets" var of the gtk+-x.0 package instead of gli…

…b to determine X11 availability

See bug #80. Thanks a lot to @rtlanceroad for reporting this & testing
the fix.

GNUmakefile

@@ -70,20 +70,17 @@ endif
 # If no GTK_VERSION is set, try to auto-determine, with GTK 3 preferred
 ifeq ($(GTK_VERSION), 0)
 	ifeq ($(shell $(PKG_CONFIG) --errors-to-stdout --print-errors "$(LIBS_GTK3)"), )
-		LIBS=$(LIBS_GTK3)
-		GDK_LIB=gdk-3.0
+		override GTK_VERSION=3
 	else
 		LIBS=$(LIBS_GTK2)
-		GDK_LIB=gdk-2.0
+		override GTK_VERSION=2
 	endif
 endif
 ifeq ($(GTK_VERSION), 2)
 	LIBS=$(LIBS_GTK2)
-	GDK_LIB=gdk-2.0
 endif
 ifeq ($(GTK_VERSION), 3)
 	LIBS=$(LIBS_GTK3)
-	GDK_LIB=gdk-3.0
 endif
 LIBS+=$(LIBS_GENERAL)
 
@@ -96,7 +93,7 @@ else
 endif
 
 # We need X11 to workaround a bug, see http://stackoverflow.com/questions/18647475
-ifeq ($(filter x11, $(shell pkg-config --errors-to-stdout --print-requires-private $(GDK_LIB))), x11)
+ifeq ($(filter x11, $(shell pkg-config --errors-to-stdout --variable=target gtk+-$(GTK_VERSION).0; pkg-config --errors-to-stdout --variable=targets gtk+-$(GTK_VERSION).0)), x11)
 	LIBS+=x11
 endif