GPX 2.1 alpha -- an unofficial GPX release

These are unofficial GPX 2.1 alpha releases which incorporate some fixes as well as some changes. See the commit history for what these all entail.

The builds for Windows do NOT include USB / serial IO support. That because making a single executable with no DLL dependencies is more time than I care to invest. GPX can be built with USB / serial IO support on Windows with Cygwin, but that then requires providing a number of DLLs with the executable. I've built these Windows executables with MinGW but without USB / serial IO support. (Lack of POSIX termios support being the issue.)