Make the build reproducible

Make the build reproducible

Whilst working on the Reproducible Builds effort [0] we noticed
that re2c could not be built reproducibly.

This is because it used the current build date in the manual page and was
originally filed in Debian as #934697 [1].

This patch uses the SOURCE_DATE_EPOCH [2] environment variable.

 [0] https://reproducible-builds.org/
 [1] https://bugs.debian.org/934697
 [2] https://reproducible-builds.org/specs/source-date-epoch/

Signed-off-by: Chris Lamb <lamby@debian.org>