It was intended that the C compiler type string for MSVC would eventually have
the form "msvc_<n>" (where <n> is the version number). The configure script is
still generating the older form of string, "cl_<n>"; we want to avoid this
because it's too easy to confuse with clang. Make configure generate the new
kind of C compiler type string.
configure.ac:
Generate the new sort of compiler type strings for MSVC.
scripts/mgnuc.in:
compiler/globals.m:
Do not recognise the old sort any longer.