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