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.
Whitespace change that replaces introduced tab with spaces.
Support more arbitrary sibling/subdir build directories for opm-parser.
Removed printf debugging messages
Do not use find_package in config mode for opm-parser.

Currently opm-parser does not create opm-parser-config.cmake files. Therefore
building with duncontrol fails as it sets opm-parser_DIR. With this patch
we force CMake to always use module mode for finding which fixes this.