build-package.sh: compatibility with "reproducible builds"

Though git method of getting persistent timestamp won't work in submodules
and snapshots and will fallback to current date.

build-package.sh

@@ -3,7 +3,7 @@
 
 set -e -o pipefail -u
 
-SOURCE_DATE_EPOCH=$(date "+%s")
+SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct 2>/dev/null || date "+%s")
 export SOURCE_DATE_EPOCH
 
 : "${TMPDIR:=/tmp}"