Force use of compatibility compilers on Apple's command line tools

The cc and c++ compilers included in Apple's command line tools show some
unusual behaviour which causes the build to fail in several projects under
Xcode 4.3

This patch uses the gnu compatibility gcc and g++ compilers on OS X instead
to reduce problems, and generates an error when an llvm based compiler is
detected because they currently do not generate usable emulator executables
due to them not honouring global register variables.

Change-Id: I506c22dad3dcbd41df3c7672802a675d3655e262
Signed-off-by: Al Sutton <al@funkyandroid.com>