When trying to match full path libraries on the mpicc link line, the
current regex is not strict enough and improperly matches a partial path
to the Xcode compiler path with homebrew mpich.  This adjusts the regex
to look for a space preceeding the library name to prevent the first
argument, i.e. the compiler invocation, from being matched, and require
either a space or end-pattern after the library to prevent mismatches with
arguments that contain the library suffix but don't actually end with it.