Allow for selecting the pkg-config executable

Over at http://github.com/spacetelescope/sphere , I need to link against the `qd` library for double-double and quad-double arithmetic.  It doesn't supply a pkg-config .pc file, but it does supply its on `qd-config` script which is API compatible with `pkg-config`.  By supporting a custom executable for `pkg-config` (instead of just hard coding it to `pkg-config`), we can easily use this script and find out where the library is.