This allows the user to override the directory for the installation
of the pkg-config files (from the default LIBDIR/pkgconfig).

It follows the usual behaviour of Makefiles generated by automake.
Signed-off-by: Ingo Brückl <ib@wupperonline.de>
Reviewed-by: Andreas Cadhalpun <andreas.cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>