On macOS work around broken compilers by disabling inheriting CC & CXX

* Normally we want to inherit CC and CXX from the parent process to ensure that
  the user's choice of compiler is respected. Unfortunately macOS 10.14 starts
  shipping broken compilers, so inheriting CC & CXX causes the swig compilation
  to fail.
* As a workaround, we just disable this on macOS assuming the user won't ever
  want to override CC & CXX because it's all locked into XCode anyway.