* We also only use GIT_COMMIT_HASH where necessary to avoid rebuilding many
files for no reason, including splitting version.cpp out into a separate
project as we do with VS since otherwise changing its preprocessor defines
rebuilds the whole renderdoc project.
* At the same time, move the git hash to be internal only so we don't have to
try to link version.cpp into other projects like renderdoccmd or qrenderdoc.
* On windows, the change in a global GIT_COMMIT_HASH define in each
project needing it meant a full rebuild every time the commit changed.
* Ideally we'd set the define only on one file in each project, but
MSBuild doesn't seem to support that. Instead we make a new tiny
project that compiles a single cpp exporting a global var, and
reference that global var in each other project.
baldurk