This is useful for developers who may have multiple builds of the game
from various different branches and may easily forget which build of the
game is what.
This shows up in the bottom-right corner of the title screen and also
with the `-version` command-line option, and in the status message
printed when building the game.
The commit hash is now properly updated every time it gets changed, and
not just when CMake gets re-ran.
For this to work, we need to use a few CMake tricks. We add a custom
target with ADD_CUSTOM_TARGET(), which is apparently always considered
out-of-date (but I had to add a BYPRODUCTS line to get it to actually
work), and we use the target to run a .cmake file every time we build.
Also, VVVVVV needs to depend on this custom target, to ensure that the
game gets built AFTER the version gets generated - otherwise there'll be
an error. So we do an ADD_DEPENDENCIES() after the ADD_EXECUTABLE() for
VVVVVV.
This file, version.cmake, is just the Version.h.out generation that I
added previously, but the important thing about all of this is that if
the contents of Version.h.out doesn't change, and thus if the commit
hash hasn't changed, then CMake will never recompile and relink anything
at all. (At least with the Ninja generator.)
On a small note, since the Version.h.out generation is now a separate
script that is guaranteed to get ran on every single build, while the
Git FIND_PACKAGE() gets ran only at configure time, it is possible for
the cached path of the Git executable to get out of date. Fixing this
requires a simple re-configure (ideally), but in case it wasn't fixed,
the INTERIM_COMMIT and COMMIT_DATE variables would get set to empty
strings instead of containing a value. To prevent this from happening,
I've removed ERROR_QUIET from the EXECUTE_PROCESS() calls in
version.cmake, because it's better to explicitly error if the Git
executable wasn't found than implicitly carry on like nothing happened.
