Cmake build: Moved creation of variable LYX_GIT_COMMIT_HASH to development/cmake/scripts/LyXGetVersion.cmake.

1.) The only file which needs this define is src/version.cpp. Our use in config.h
resulted in recompiling most of our sources.
With this change, only version.cpp needs to be recompiled.

2.) Without reconfiguring, the old commit hash was still valid. Now the
detection of commit hash is done at compile time.
Forgot this file in the last commit