TRAVIS CI updated to include Windows and osx testing

TRAVIS CI updated to include Windows and osx testing

TRAVIS CI added Windows to their platform list in late 2018.

Update the .travis.yml file to include a pair of Windows in
machines in the testing suite, one built with visual studio
command line tools and the other with mingw gcc tools.

The visual studio build is currently using nmake with the
sys/winnt/Makefile.msc Makefile from our distribution,
That's the same process we've been using for building
our binaries, pretty much.

BRH may be able to modernize it over the next couple of
weeks to use the msbuild process instead.

I went with the HINTS environment variable on windows
for consistent self-documenting purposes, even though
the environment variable isn't used on windows.

included:
   os: linux
   Compiler: gcc C
   HINTS=linux

   os: linux
   Compiler: clang C
   HINTS=linux

   os: linux
   Compiler: gcc C
   HINTS=linux-x11

   os: linux
   Compiler: gcc C
   HINTS=linux-qt5

   os: linux
   Compiler: gcc C
   HINTS=linux-minimal

   os: windows
   language: shell
   HINTS=Windows-visual-studio

   os: windows
   HINTS=Windows-mingw

excluded:
   os: osx
   Compiler: clang
   Xcode: xcode10.2 C
   HINTS=macosx10.14