This is required for detecting ar and ranlib properly, required
e.g. when cross compiling for android from OS X.

One can't set CMAKE_AR or CMAKE_RANLIB directly. Normally they
should be set properly automatically within cmake, but those
codepaths seem to be skipped when we set the C/CXX compilers
manually.
Signed-off-by: Martin Storsjö <martin@martin.st>