The patch "compositor-android: fix build flags" started using GCC_CFLAGS
for C++ files, too. That lead to the following warnings when building a
C++ file:

cc1plus: warning: command line option "-Wstrict-prototypes" is valid for
Ada/C/ObjC but not for C++
cc1plus: warning: command line option "-Wmissing-prototypes" is valid
for Ada/C/ObjC but not for C++

Introduce GCC_CXXFLAGS, similar to GCC_CFLAGS, but for C++, avoiding the
problematic compiler flags.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>