allow building with ICL on Windows if you have it installed into MSVC [Dean London]

The ICL compiler on Windows installs over MSVC and piggybacks onto the build tools Microsoft provide.  To enable compilation use USE_ICL=1 when compiling with MSVC_BUILD=1 (from the ICL enabled command prompt)

 + gives a more stable build than the MSVC compiler
 + more informative compile errors when using ICL compared to MSVC compiler
 + can still use MSVC debugging tools (better than GCC)
 + can use additional intel tools etc.
 - build is still slower than GCC one

out of whatsnew:
I've cleaned up this submission a fair bit, could maybe be cleaned a little more, but it's much better than what I was sent.  Looking at the list of additional warnings that ICL requires to be disables reveals some to be mundane (and could probably be fixed in the source, eg. the way it doesn't like the hng64.c commenting style) whereas others look like they might be legitimate (and useful) warnings about problems we should look into.

I briefly tested this with VC2013 + Patch 1 RC and the current ICL, it appears to build and run fine.