Use SOURCE_DATE_EPOCH for timestamp if present

The Debian reproducible builds effort suggests using
this environment variable for timestamps.

cmake/formatdate.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python
+from __future__ import print_function
+import os
+import sys
+import datetime
+
+def usage():
+    print("Usage:", os.path.basename(sys.argv[0]), "<seconds from epoch>")
+
+if len(sys.argv) != 2:
+    usage()
+    sys.exit(1)
+
+ts = sys.argv[1]
+
+build_date = datetime.datetime.utcfromtimestamp(int(ts))
+
+print(build_date.strftime("%Y-%m-%d"))