allow the opm-parser test to fail

since the module currently calls the tests for cJSON and boost with
the REQUIRED flag set, cmake aborts completely even if opm-parser is
not REQUIRED. For modules which depend on opm-core that's currently a
non-issue because core has a requirement on parser, but the parser may
be useful for external projects as well, so we play nice.

Also, with this patch the config.h variable HAVE_OPM_PARSER is set...