Make it possible to use different toolkits by adding #ifdefs.

gtk/Makefile

@@ -1,12 +1,12 @@
 PREFIX ?= /usr/local
 
 all:
-	@objfw-compile -Wall -g --lib 0.0 -o objgui -I../headers \
+	@objfw-compile -DOG_GTK -Wall -g --lib 0.0 -o objgui -I../headers \
 		`pkg-config --cflags --libs gtk+-3.0` \
 		`ls *.m | fgrep -v test.m`
 
 test:
-	@objfw-compile -Wall -g -o test -I../headers \
+	@objfw-compile -DOG_GTK -Wall -g -o test -I../headers \
 		`pkg-config --cflags --libs gtk+-3.0` \
 		*.m
 

headers/OGApplication.h

@@ -1,5 +1,3 @@
-#include <gtk/gtk.h>
-
 #import <ObjFW/ObjFW.h>
 
 @protocol OGApplicationDelegate <OFObject>

headers/OGWidget.h

@@ -20,14 +20,18 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#ifdef OG_GTK
 # include <gtk/gtk.h>
+#endif
 
 #import <ObjFW/ObjFW.h>
 
 @interface OGWidget: OFObject
 {
 @public
+#ifdef OG_GTK
 	GtkWidget *widget;
+#endif
 }
 
 - (void)show;