package/matchbox-lib: correctly fix the .pc file

First, the .pc file was so far fixed as a post-configure hook of the
matchbox-fakekey package, by directly tweaking the .pc file installed in
staging by matchbox-lib. That's uterly wrong and bad.

So, we move the fix to matchbox-lib.

Second, it was incorreclty tweaking the .pc file when xlib_libXft was
not enabled, because only then a path to staging was present.

Third, even when xlib_libXft was enabled, the tweaking was still wrong,
because unnecessary.

Fix all that.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>