Make sure PKG_CONFIG_PATH is set according to target libs

Make sure PKG_CONFIG_PATH is set according to target libs

- pkg-config isn't multilib-aware by itself so things fail when
  building packages for secondary archs (RhBug:212522)
- A variant of this patch has been in Fedora and RHEL for quite some
  time, but they used to just brutally override whatever might
  have been in PKG_CONFIG_PATH, which can break things in a custom
  setup (RhBug:569773). Append instead of full override.
(cherry picked from commit aa8979f4d1cd5ad5b2c69135595d90489e039817)