Search siblings before system directories

If there is a sibling directory with the name of the module we are
searching for, it is probably part of the same suite, and is the
version we intend to use, before the system version.

This behaviour can be altered with the option SIBLING_SEARCH.

Note, however, that if the sibling is a source tree, it cannot guess
which subdir the build tree is in. You will then probably end up
with headers from the sibling and libraries from the system!