Fix the namespace cleanliness check on MinGW64.

configure.ac:
	Check for the host specific version of the nm tool.
	For MinGW64 we need to use the MinGW64 version, not
	the MinGW32 version (i.e. x86_64-w64-mingw32-nm, not
	nm).

Mmake.common.in:
	Use the host specific version of the nm tool to list
	external symbols in object files.