configure.in:  Don't do MULTILIB_HACK on non-Precise hosts
rtapi/Submakefile:  fix race condition creating links for parallel build
Makefile, Makefile.inc:  fix EMC2_RTLIB_DIR installation location

In the top-level make, the flavor is arbitrarily set to the first
detected.  Installing all flavors into EMC2_RTLIB_DIR is therefore
incorrect; the top-level, flavor-agnostic, common directory is the
right location.