Consider to delete MDBX_BUILD_TIMESTAMP

[AskAlexSharov]

There is no logic based on this constant.

But it breaks reproducibility of app builds: shasum -a256 mybinary is different from build to build only because of MDBX_BUILD_TIMESTAMP.
Reproducibility checksum useful for security reasons - to check that no malware code was embedded into app sources during "release binary artifacts" process.

[erthink]

It was assumed that the ability to predefine MDBX_BUILD_TIMESTAMP for builds reproducibility would be added.
Looks like it's time to do it.

[AskAlexSharov]

It works:

MDBX_BUILD_TIMESTAMP=unknown make config.h
make mdbx-static.o