Define GLIB_COMPILATION when building libgthread.so

This is necessary, because gthreadprivate.h can only be
included inside GLib, otherwise single-include guards trigger.

gthread/Makefile.am

@@ -7,6 +7,7 @@ DIST_SUBDIRS = tests
 AM_CPPFLAGS = 				\
 	$(glib_INCLUDES) 		\
 	-DG_LOG_DOMAIN=\"GThread\" 	\
+	-DGLIB_COMPILATION		\
 	@GTHREAD_COMPILE_IMPL_DEFINES@ 	\
 	@GLIB_DEBUG_FLAGS@ 		\
 	-DG_DISABLE_DEPRECATED