use the same search path as the pkg-config normally used on these systems:
Debian Linux Debian kFreeBSD RedHat/CentOS/Fedora Linux FreeBSD* NetBSD OpenBSD
FreeBSD 10 doesn't support PKG_CONFIG_DEBUG_SPEW so I can't verify the path there, but the path is correct for FreeBSD 9
Non .deb or .rpm based Linux distributions are not detected and will use the default non-platform specific search path. .rpm based distributions other than RedHat/CentOS/Fedora may be misidentified as one of those. Patches to correct unidentified or misidentified platforms would be eagerly received.
this is a significant change in behavior, but arguably a useful one
add file_path option for loading specific .pc file