python: don't install bottles on Xcode-only systems

Python's setuputils remembers the flags with which Python is built. When users later do `pip install foo` (for some package foo containing C code), foo is built with those same flags.

For some packages, the correct flags depend on whether the Xcode CLT are installed. `Python.distutils_fix_superenv`, which is called from `install`, has logic to do the right thing, but this logic is bypassed when bottles are installed because the bottlers all have CLT.

homebrew/homebrew-python#141 is an example of this problem surfacing. This should be fixed in the python formula so that people can `brew install python` and then `pip install pillow` without CLT.

It will also be fixed in the homebrew-python/pillow formula so that users can `brew install pillow` with system Python.