Allow to override build date

Allow to override build date

Summary:
Allow to override build date
for distribution packages that use autoreconf
to recreate the configure script

See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Note: This patch will work with GNU date. If BSD date support is
important, a more complex patch needs to be done.

http://rb.zq1.de/compare.factory-20170713/efl-compare.out
also shows other diffs in .edj files that still remain

@fix

T5495

Reviewers: cedric

Subscribers: stefan_schmidt, jayji, netstar, jpeg

Maniphest Tasks: T5495

Differential Revision: https://phab.enlightenment.org/D5023