gcc: Backport fixes for -W parentheses in C++

This fixes GCC 19564:
  http://gcc.gnu.org/ml/gcc-patches/2006-12/msg00868.html
  http://gcc.gnu.org/ml/gcc-patches/2006-12/msg01772.html
  http://gcc.gnu.org/ml/gcc-patches/2007-01/msg00545.html

The patch and its fixes are used by Google in their enhanced gcc-4.2.1
port and predates the license switch in GCC so they are still under
GPLv2.

MFC after:	3 weeks