Support multiple SDK directories

Via the SDKPATH, which is like every other PATH variable.
Also raises an EnvironmentError if it can’t find the designated
SDK.
Use “gcc” for the compiler name when Xcode is 4 or later.

This is a standard link to llvm-gcc-4.2 in Xcode 4 and to clang
in Xcode 5.

Also, raise an EnvironmentError if you try to build against the 10.4u
SDK with Xcode 4 or later: SDK 10.4u doesn’t work with gcc-4.2, and
only Xcode 3 and earlier support gcc-4.0.