Build: fix #865 yet again

Build: fix #865 yet again

It appears that the symmetry changes of setting CC and CFLAGS correctly
for C code compilation in commit 104f02f broke things for people using
some other version of setuptools which uses those vars instead of
CXX and CXXFLAGS when compiling C++ code. In order to make this work
everywhere, set _both_ sets of variables as required for C++ compilation.
No C code is compiled by the python binding module anyway.