GraphicsMagick CFLAGS detection

Currently, dvdauthor fails to build against GraphicsMagick as it
doesn't retrieve the CFLAGS from pkg-config (see [0]). The attached
patch simplifies configure.ac, using the pkg-config macro to obtain
the settings for GraphicsMagick.

[0] https://bugs.gentoo.org/show_bug.cgi?id=459976
--
Andii :-)