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
I realize this hasn't changed, but it looks like a directory is created and then immediately deleted. Is that necessary? Passing `-f` to `rm` will ensure it succeeds regardless of whether the directory exists or not.