dumper: Attempt to identify MSVC/'cl' more precisely

Checking for 'cl' in the whole compiler string trips up people who do
CC='gcc --command-with-cl-in-the-name'.  We should (probably) only
look at the first argument, although this will break for people doing
CC='ccache cl', but would someone do that?

See: https://bugzilla.gnome.org/show_bug.cgi?id=698090