discover.ml tries to guess where corresponding library and include
directories reside. It does this by assuming the "include" and "lib"
portions are always at the end and concatenating a relative path to
get to the other. This works well in most cases. However PKGSRC has an
instance where this isn't true.

This patch finds the "include" and "lib" portions within the path and
replaces them. This keeps it backwards compatible, but adds PKGSRC
support.
Since the PKGSRC project can be found on many, many Unix-like
platforms, this patch adds the base directory used by PKGSRC to
discover.ml's search path.