Add a variable to force which .pc to use
add libxul (aka xulrunner 1.9) before iceape-plugin and xulrunner-plugin (xul 1.8)
Don't depends on mozilla-config.h when using pkg-config. It's the old way
of doing thing and requires us to depends on the foo-xpcom.pc

Needs backporting once tested
(cherry picked from commit 26c5015f)
Signed-off-by: Christophe Mutricy <xtophe@videolan.org>