Fix generating demo_conf.h on builds without GIT in PATH

Hi,

The latest update to how demo_conf.h is generated assumes that the git program is in the PATH, which may not always be the case, so this adds a fallback like we did before if the git program was not found. Also, pass in the $(srcroot) to the script so that we can have call git on the source directory, so that we can get the right SHA1 sum (or nothing in stdout if $(srcroot) is not a GIT checkout). Also do a minor cleanup to the lines where we output the line for #define VCS_TAG ....

With blessings, thank you!