Link test exes statically to libgcc, libstdc++ for MinGW

Also fix the GetExecutablePath test when using MinGW. Statically linking the stdlibs makes the tests a bit easier to run in cross-compilation from Linux (via wine) or Cygwin.

I was experimenting with running the cross-compile and tests as part of a build matrix on Travis here https://travis-ci.org/tkelman/mp/jobs/34588478 - there's a spurious failure in solver-test.cc that's being too picky on the "No such file or directory" error message. I've seen "File not found." and also "The system cannot find the file specified." for that error message, and I imagine the message could also be locale-dependent. Not sure what the failures in ASLBuilderTest and TableProxyTest are caused by.