Clean up find_msms()

...incorporating insight documented in the commit note of
c9c2e27c95a87aa75513555ef485cac978889b19 "Adapt for Visual Studio 2013," and
cloph's idea to use Perl to read that oddly named Windows env var in cygwin.

Change-Id: I55d364f7a24b50d171a7fdca000ca2d747f6db20