Fix pkg-config not being used for OpenSSL on non-FreeBSD systems.
Add a work around for trampling over PKG_CONFIG_PATH.

The extend_pkg_path method will be removed in 2.2 as it
causes problems with non-standard pkg-config paths. This
commit adds a workaround until that happens.
Merge pull request #900 from SaberUK/insp20+build-system-fixes

[2.0] Fix various problems with the build system.
Fix unit-cc not loading configure cache when building files.
Merge pull request #910 from SaberUK/insp20+even-more-build-system-fixes

Fix unit-cc not loading configure cache when building files.