Make sane reproducible

The reproducible builds project aims to make software reproducible allowing users to reproduce distributed binaries and packages locally. Sane records the build date in backends/tools/sane-desc.c to solve this either the time is stripped as the patch in Debian does. Or use SOURCE_DATE_EPOCH to provide a predictable value for the time when rebuilding or as alternative rely on GCC and use __DATE__, __TIME__ which is reproducible thanks to GCC supporting SOURCE_DATE_EPOCH

Unreproducible results can be viewed here: https://tests.reproducible-builds.org/archlinux/extra/sane/sane-1.0.28-3-x86_64.pkg.tar.xz.html