build: Include Cocoa/Carbon in Libs.private if targetting Mac OS X

This allows static linking against GIO on OS X, which would previously
fail due to unresolved symbols from the above two frameworks.

https://bugzilla.gnome.org/show_bug.cgi?id=668152

configure.ac

@@ -213,13 +213,17 @@ AM_CONDITIONAL(MS_LIB_AVAILABLE, [test x$ms_librarian = xyes])
 
 AS_IF([test "x$glib_have_carbon" = "xyes"], [
   AC_DEFINE(HAVE_CARBON, 1, [define to 1 if Carbon is available])
-  LDFLAGS="$LDFLAGS -Wl,-framework,Carbon"
-])
+  CARBON_LIBS="-Wl,-framework,Carbon"
+  LDFLAGS="$LDFLAGS $CARBON_LIBS"
+], [CARBON_LIBS=""])
 
-if test "x$glib_have_cocoa" = "xyes"; then
+AS_IF([test "x$glib_have_cocoa" = "xyes"], [
   AC_DEFINE(HAVE_COCOA, 1, [define to 1 if Cocoa is available])
-  LDFLAGS="$LDFLAGS -Wl,-framework,Foundation"
-fi
+  COCOA_LIBS="-Wl,-framework,Foundation"
+  LDFLAGS="$LDFLAGS $COCOA_LIBS"
+], [COCOA_LIBS=""])
+
+AC_SUBST([COCOA_LIBS])
 
 dnl declare --enable-* args and collect ac_help strings
 AC_ARG_ENABLE(debug,