CMP0047: Fix CMAKE_COMPILER_IS_GNU(CC|CXX) in OLD behavior

The implementation of CMP0047 detects the QNX compiler id internally,
then changes it to GNU after the language is fully enabled.  This is too
late to correctly set the old IS_GNU variables, so set them as part of
the policy OLD behavior.

Reported-by: Mike Lattanzio <mlattanzio@blackberry.com>
Merge topic 'fix-CMP0047-IS_GNU-vars' into next

70796ef0 CMP0047: Fix CMAKE_COMPILER_IS_GNU(CC|CXX) in OLD behavior