Attempt to handle isysroot SDK options when compiling CC backend

The CommonCrypto backend is presumed available if the OS you're
compiling on is 10.8+. Unfortunately, if Python is built against an
older SDK (such as the official distribution from python.org) this
assumption is inaccurate. This patch attempts to detect the presence of
an isysroot flag in the python CFLAGS and parse the SDK version in use.
If it works it should fix #1277.