Make it possible to have implementations in different toolkits.

Also, make it possible to share the headers.

gtk/Makefile

@@ -1,18 +1,18 @@
 PREFIX ?= /usr/local
 
 all:
-	@objfw-compile -Wall -g --lib 0.0 -o objgui \
+	@objfw-compile -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 \
+	@objfw-compile -Wall -g -o test -I../headers \
 		`pkg-config --cflags --libs gtk+-3.0` \
 		*.m
 
 install:
 	mkdir -p ${PREFIX}/include/ObjGUI
-	cp *.h ${PREFIX}/include/ObjGUI/
+	cp ../headers/*.h ${PREFIX}/include/ObjGUI/
 	cp libobjgui.so ${PREFIX}/lib/libobjgui.so.0.0
 	ln -sf libobjgui.so.0.0 ${PREFIX}/lib/libobjgui.so.0
 	ln -sf libobjgui.so.0 ${PREFIX}/lib/libobjgui.so