These were accepted in https://github.com/alexcrichton/gcc-rs/pull/212, but never actually worked and weren't being tested. This fixes the logic errors that prevented the new style from working, and also updates the tests to use the new style.
It allows one to add a flag to the compiler on the condition that the
compiler supports it.
We check if the compiler supports the flag by trying to compile a
simple executable. If there is some output on stderr we consider that
the compiler failed and the flag is not supported.
Visual Studio 2015, recently releases, includes the Universal CRT, a different
flavor than was provided before. The binaries and header files for this library
are included in new locations not previously known about by gcc-rs, and this
commit adds support for the necessary probing to find these (for now).