libmakepkg: add routine for linting $SOURCE_DATE_EPOCH

This can only ever be an int, and the specification states that a
malformed timestamp should be considered a fatal error.

https://reproducible-builds.org/specs/source-date-epoch/

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>