configure: make $targetos lowercase, use windows instead of MINGW32

targetos is already mostly the same as Meson host_machine.system(),
just in CamelCase.  Adjust Windows, which is different, and switch to
lowercase to match Meson.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
1 file changed