The goal of reproducible builds is that a rebuild of the same source
code with the same compiler, libraries, etc. should result in the same
binaries. SOURCE_DATE_EPOCH provides a standard way for build systems
to fill in the date of the latest source change, typically from a git
commit or from metadata like the debian/changelog in Debian packages.

This does not change anything if SOURCE_DATE_EPOCH is not defined;
the intention is that a larger build system like a Debian package
will define it.

Please see https://reproducible-builds.org/ for more information about
reproducible builds.

Originally sent to ioquake3 as commit
ioquake/ioq3@9c76b54
(which also touched engine files not present here).

Signed-off-by: Simon McVittie <smcv@debian.org>

With a suitable setting for SOURCE_DATE_EPOCH, this makes building
qagame.qvm reproducible too.

Signed-off-by: Simon McVittie <smcv@debian.org>