Optimize compile script on MSYS.

Optimize compile script on MSYS.

* lib/compile (func_file_conv): Add new argument 'lazy' which
takes an optional list of conversion types where the requested
conversion isn't needed.
(func_cl_wrapper): Take advantage of the above for cases where
MSYS is doing the conversion for us.
Suggested by Ralf Wildenhues.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>