manywarnings: Add support for C++. * build-aux/g++-warning.spec: New file. * m4/manywarnings-c++.m4: New file. * m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC(C++)): New macro, that redirects to manywarnings-c++.m4. * modules/manywarnings (Files): Add m4/manywarnings-c++.m4.