Use the toolchain from PATH on Windows

We don't want to fetch the compiler path from the mkspec on Windows
the same way that we do on linux/embedded. Qt usually let the build
environment decide the MSVC toolchain, as it is usually done on
Windows.

This patch also passes the ninja_use_custom_environment_files ninja
generator option to make sure that gyp doesn't try to construct the
build environment by running the env scripts itself.

Change-Id: I6e04471eb994b81a9f1b8b5f149b4dbddbb320a8
Reviewed-by: Andras Becsi <andras.becsi@digia.com>