Use new-style C compiler type for MSVC.

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.