ICL c99wrap support

ICL on windows is command line compatible with msvc, in this case
c99wrap serves as a convenience factor simply to make things easy when
calling it from places such as libav's configure/build scripts.

To maintain consistency with other use cases it will call the converter
by default.  When using ICL normally it should be called like:

    c99wrap -noconv icl ...

You cannot use -keep and -noconv at the same time for obvious reasons
(you can't keep temporary files that don't exist).