Introduce CC/CXX/JAVAC_WRAPPER to wrap the calls to the compiler.

This is to make it possible to wrap the compiler invocations with
custom wrappers (e.g., distcc/goma) by setting the CC_WRAPPER,
CXX_WRAPPER, JAVAC_WRAPPER variables in the build environment (without
having to know in advance the path to the compiler)

Change-Id: Ide800c24f0c2ebbb1cfb358bd8f99ec8a9d41253