buildsystem: add missing pkg-config stub for libweston uninstalled

buildsystem: add missing pkg-config stub for libweston uninstalled

-uninstalled.pc files are a pkg-config facility for working with
uninstalled libraries.

With pkg-config, foo-uninstalled.pc overrides foo.pc. foo-uninstalled.pc
should never be installed, and will be generated with references to the
build directory.

If you set up your environment so pkg-config looks for .pc files in your
build directories, you can use this to build and link against libraries
you haven't installed with "make install".

This can save time and space over installing with a prefix.

Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Derek Foreman <derekf@osg.samsung.com>