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Use cmake TIMESTAMP for BUILD_DATE+TIME

Use cmake TIMESTAMP for BUILD_DATE+TIME
this simplifies code a lot
and even makes it more portable to other platforms

TIMESTAMP is available since cmake-2.8.11 ; blender already requires
cmake>=3.5 so that is fine.

Note that with CMake>=3.8, if defined, the SOURCE_DATE_EPOCH envvar
will be used by CMake here.

Reviewers: mont29, campbellbarton

Reviewed By: mont29, campbellbarton

Differential Revision: https://developer.blender.org/D5760