3742eda7

 introduced a wrong behavior
where the install commands for pkgconfig files were blindly ignoring
$(prefix). This obviously breaks the build and introduces the
possibility of the pkgconfig files not being installed to the system
at all since $(libdir) is not suposed to have a leading /, that is
$(prefix)'s job. All this resulted in a bad makefile being shipped
in the 3.9.0 release.

Signed-off-by: Filipe Laíns <lains@archlinux.org>