Use default cc to build CPython

There is compiler issue on OS X like #148. To avoid choosing older `gcc` (e.g. `gcc-4.2` on OSX, with brew's `apple-gcc42` formula), just use default `cc` as `CC` to build CPython.

Although, there is still a chance to use other `CC`. Just setting `CC` (e.g. `env CC=gcc-4.2 pyenv install -v 2.7.6`) will allow you to build with specified compiler.