don't assume pkg-config is in PATH

pkg-config might not be in path, so $PKG_CONFIG should be used instead. This is already done in other places in configure.ac, this patch just fixes the rest of the invocations to use it as well.

This, as well as AC_USE_SYSTEM_EXTENSIONS (as documented at https://github.com/collectd/collectd/issues/351#issuecomment-19444142) and the already-merged 1a822486f40287f552890a553181fd620c662ada are enough patches to build 5.4.1 on OmniOS (which doesn't ship pkg-config, but I have my own copy outside PATH)