some extconf.rb checks -I option. but cl accepts both -I and /I option. and CFLAGS in .pc(GTK+win32) is /I style.
extconf.rb needs to checks -I and /I.
gobject-2.0.pc has not CFLAGS. but extconf.rb checks gobject-2.0.pc for gsignal.h.
if CFLAGS in gobject-2.0.pc is blank, extconf.rb needs to use CFLAGS in glib-2.0.pc instead of gobject-2.0.pc.