Merge libgdk and libgtk

This commit does a number of things:
- remove some dead wchar configury from configure.ac and gdkconfig.h
- repurpose gdkconfig.h as header that contains GDK_WINDOWING_foo
  macros for each included backend, include it in gdk.h and install
  it in $includedir instead of below $libdir
- drop the backend from the library names
- build libgdk-3.0.la as a convenience lib and include it in libgtk-3.0.la
It does not yet enable building multiple backends at the same time.

demos/Makefile.am

@@ -11,14 +11,12 @@ INCLUDES = 				\
 	$(GTK_DEBUG_FLAGS)		\
 	$(GTK_DEP_CFLAGS)
 
-DEPS = 									\
-	$(top_builddir)/gdk/$(gdktargetlib) 				\
-	$(top_builddir)/gtk/$(gtktargetlib)
+DEPS = \
+	$(top_builddir)/gtk/libgtk-3.0.la
 
-LDADDS = 								\
-	$(top_builddir)/gdk/$(gdktargetlib)        			\
-	$(top_builddir)/gtk/$(gtktargetlib)				\
-	$(GTK_DEP_LIBS)							\
+LDADDS = \
+	$(top_builddir)/gtk/libgtk-3.0.la	\
+	$(GTK_DEP_LIBS)				\
 	$(MATH_LIB)
 
 noinst_PROGRAMS = 		\

demos/gtk-demo/Makefile.am

@@ -55,13 +55,11 @@ INCLUDES = \
 	$(GTK_DEP_CFLAGS)
 
 DEPS = \
-	$(top_builddir)/gdk/$(gdktargetlib) 				\
-	$(top_builddir)/gtk/$(gtktargetlib)
+	$(top_builddir)/gtk/libgtk-3.0.la
 
 LDADDS = \
-	$(top_builddir)/gdk/$(gdktargetlib)        			\
-	$(top_builddir)/gtk/$(gtktargetlib)				\
-	$(GTK_DEP_LIBS)							\
+	$(top_builddir)/gtk/libgtk-3.0.la	\
+	$(GTK_DEP_LIBS)				\
 	-lm
 
 bin_PROGRAMS = gtk3-demo