Don't put relative paths in pkg-config files, including uninstalled
ones. For those, use @abs_topbuilddir@ and @abs_topsrcdir@ as we
do elsewhere.
Remove libraries= directives, which doesn't seem to be a pkg-config
variable that actually exists, but has been in all our pkg-config
files for as long as they've existed.
It's architecture dependent and should not be placed into the include
directory as the assumption is that all those headers are architecture
independent.
https://bugzilla.gnome.org/show_bug.cgi?id=739767
