Updating re2 build to use -mmic for intel+knc builds

Following the lead of other third-party Makefiles, if we're building for
KNC using the intel compiler, throw the -mmic flag when building re2.
While here, I did some other cleanup:

* broke a long line of commands across several lines
* passed CXX into both recursive make steps just for symmetry (I don't
  believe it's used for the install step)
* removed the set of CC as it doesn't seem to be used by re2's Makefile