Make build reproducible

[jelly]

The current libcec cannot be built twice on different machines since it records the buildtime in the binary. A solution is to honour SOURCE_DATE_EPOCH which is described on the reproducible builds website https://reproducible-builds.org/

[bmwiedemann]

in openSUSE we workaround it with
https://github.com/bmwiedemann/openSUSE/blob/master/packages/libc/libcec/libcec-build-compare.patch

but since it is cmake, one can use the TIMESTAMP macro to be more portable and provide reproducible builds at the same time. See #487 for that.

There are also build user, host and kernel version.