GDB now requires a C++ compiler

FYI,

Building GDB master now requires a C++ compiler.  The option to
build with a C compiler has been removed.

IOW, we can now make use of C++-only features in master.

Please keep in mind that we'll be wanting to backport patches
to the 7.12 branch, which supports building as C, though.  So if a
patch is likely to need to be backported, then it's going to be
easier to write it in C from the start.

More info in the conversion project's page here:

 https://sourceware.org/gdb/wiki/cxx-conversion

Thanks,
Pedro Alves