The previous implementation of showing the commit hash on the title
screen used a preprocessor definition added at CMake time to pass the
hash and date. This was passed for every file compiled, so if the date
or hash changed, then every file would be recompiled. This is especially
annoying if you're working on the game and switching branches all the
time - the game has at least 50 source files to recompile!
To fix this, we'll switch to using a generated file, named
Version.h.out, that only gets included by the necessary files (which
there is only one of - Render.cpp). It will be autogenerated by CMake
(by using CONFIGURE_FILE(), which takes a templated file and does a
find-and-replace on it, not unlike C macros), and since there's only one
file that includes it, only one file will need to be recompiled when it
changes.
And also to prevent Version.h.out being a required file, it will only be
included if necessary (i.e. OFFICIAL_BUILD is off). Since the C
preprocessor can't ignore non-existing include files and will always
error on them, I wrapped the #include in an #ifdef VERSION_H_EXISTS, and
CMake will add the VERSION_H_OUT_EXISTS define when generating
Version.h.out. The wrapper is named Version.h, so any file
that #includes the commit hash and date should #include Version.h
instead of Version.h.out.
As an added bonus, I've also made it so CMake will print "This is
interim commit [HASH] (committed [DATE])" at configure time if the game
is going to be compiled with an interim commit hash.
Now, there is also the issue that the commit hash change will only be
noticed in the first place if CMake needs to be re-ran for anything, but
that's a less severe issue than requiring recompilation of 50(!) or so
files.
