Allow to override buildDate with SOURCE_DATE_EPOCH

ADDITIONAL SYSTEM INFORMATION :
OS: openSUSE Tumbleweed

A DESCRIPTION OF THE PROBLEM :
It should be possible to get identical build results anytime anywhere. See https://reproducible-builds.org/ for why this is good.

When building the openjfx openSUSE Linux package, build results vary for various reasons:

- embedded timestamps - e.g. BUILD_TIMESTAMP
- filesystem readdir order - e.g. in openjfx/libdecora_sse.so
- other (needs further analysis)