[PATCH] [cmake] Makes arbitrary build subdirectories possible.

Instead of guessing the suffix or subdir of the build directory,
we now simply compare PROJECT_{BINARY,SOURCE]_DIR to detect it.
By this e.g. opm-core/opm-parallel is a possible build directory, too.
Merge pull request #113 from blattms/make-arbitrary-sibling-possible

[PATCH] [cmake] Makes arbitrary build subdirectories possible.