/docs/source-date-epoch/: Improve the CMake documentation

STRING(TIMESTAMP BUILD_DATE "%Y-%m-%d" UTC)

```

... works with CMake versions 2.8.11 and higher, but it only respects

`SOURCE_DATE_EPOCH` since version 3.8.0. If you do not have a modern CMake but

need reproducibility you can use the less-preferred variant:

... will compile with CMake versions 2.8.11 and higher ([released May 2013](https://cmake.org/pipermail/cmake/2013-May/054792.html)), but it only respects `SOURCE_DATE_EPOCH` since version 3.8.0 ([April 2017](https://cmake.org/pipermail/cmake-developers/2017-April/029946.html)). Note that the final argument `UTC` is required or the timestamp may vary between timezones.

If you would like to support legacy/archival versions of CMake, you can use this less-preferred variant:

```

if (DEFINED ENV{SOURCE_DATE_EPOCH})

endif ()

```

The above will work only with GNU `date`. See the POSIX shell example on how to

support BSD date.

Note that the above will work only with GNU `date`; see the POSIX shell example on how to support BSD date.

### Meson