Allow to override date

[bmwiedemann]

For openSUSE, this date ended up in pcp-debugsource rpms
via generated source files
making it differ for every build

without this patch, diff contained:

+++ /usr/src/debug/pcp-3.11.9-0.0.x86_64/src/pmclient/pmnsmap.h
  * Built by pmgenmap.sh from the file
  * pmnsmap.spec
- * on Sat Sep  3 01:48:38 UTC 2033
+ * on Wed Aug  1 12:31:06 UTC 2018

+++ /usr/src/debug/pcp-3.11.9-0.0.x86_64/src/libpcp/src/derive_parser.y     2017-11-23 12:00:00.000000000 +0000
@@ -2,7 +2,7 @@
  * DO NOT EDIT THIS FILE ... CHANGES HERE WILL BE LOST
  *
  * This file created from derive_parser.y.in (make changes there!)
- * by fix_derive_parser_y on Sat Sep  3 01:41:26 UTC 2033.
+ * by fix_derive_parser_y on Wed Aug  1 12:24:36 UTC 2018.