use pure perl PkgConfig.pm if pkg-config is not available

This does two things:

 1. makes the pure perl `pkg-config` implementation `PkgConfig.pm` a prereq if `pkg-config` cannot be found  in the PATH.
 2. use `PkgConfig.pm` if `pkg-config` is not available at runtime, preferring a real system `pkg-config`

PkgConfig.pm is being actively developed by myself (patches welcome) and is included with Strawberry Perl as of 5.20.  It is also useful on systems where pkg-config is not installed by default (which is actually almost everywhere).

It does NOT replace `Alien::Base::PkgConfig`, although we should maybe consider this down the road if there are sufficient advantages.