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