configure: Set PKG_CONFIG_PATH to get the right settings for the target host.

[Michael Müller]

Setting PKG_CONFIG_PATH doesn't seem like a good option for me. You
would need to implement the path for every possible distribution and
platform to not cause any regressions. On Arch Linux it is for example
/usr/lib32/pkgconfig. On FreeBSD it is /usr/libdata/pkgconfig/. Not to
speak of cross compilation cases in which users might have a special
build of pkgconfig where overriding PKG_CONFIG_PATH doesn't make any sense.

Regards,
Michael