Proprietary C++ compilers no longer awful

I've been surprisingly successful with the gtkmm porting work that I'm doing for The Written Word. gtkmm,
glibmm, and libsigc++ now build with the SUN Forte CC, AIX xlC, IRIX MipsPro, and Tru64 C++ compilers, as well as g++ and the Intel
compiler. libsigc++ is quite a template test case, so that's a major improvement compared to a couple of years ago. I added a few configure
compiler tests to libsigc++ and glibmm to work around the (rather obscure) differences.

And a Microsoft employee even seems to have fixed a MSVC++ bug that
was causing us problems, though we'll have to wait for the new Visual Studio version to see whether it's really what we need. Back when I was developing on Windows there was no way to even report a bug, let alone expect it to be fixed.