do not produce warning on msvc
fix finding graphviz on windows

There is no graphviz_version.h file in the windows package, so determine the version by executing the dot tool instead.
get graphviz path from the PATH variable instead of having to provide it by commandline cmake parameter
Add license headers.
Compute the version when it's new enough, not when the test program failed.

This fixes the Linux build here, and should not affect Windows.
Prepare for 1.3.1 release
gammaray.spec - fix typo in date
Merge remote-tracking branch 'github/1.3'

Conflicts:
	CMakeLists.txt
	cmake/FindGraphviz.cmake