version-generate

same as #1177.
for openh264v1.1 branch.

So version.h is now a generated file so it should not be in source control.  

I assume you want VS and XCode project files to still work so they should generate this file as well as part of their build.  I believe it's called a 'pre-build event' in VS.    This would also allow them to set the version accurately the same way as make.

I am also open to abandoning the VS projects and pushing people to use the Makefile on Windows.