Add configure option --enable-build-timestamp.

* configure.ac (BUILD_TIMESTAMP): Set to "<none>" by default.
--

This is based on
libgpg-error commit d620005fd1a655d591fccb44639e22ea445e4554
but changed to be disbaled by default.  Check there for some
background.

Signed-off-by: Werner Koch <wk@gnupg.org>

configure.ac

@@ -260,11 +260,21 @@ changequote([,])dnl
 BUILD_FILEVERSION="${BUILD_FILEVERSION}mym4_revision_dec"
 AC_SUBST(BUILD_FILEVERSION)
 
+AC_ARG_ENABLE([build-timestamp],
+  AC_HELP_STRING([--enable-build-timestamp],
+                 [set an explicit build timestamp for reproducibility.
+                  (default is the current time in ISO-8601 format)]),
+     [if test "$enableval" = "yes"; then
         BUILD_TIMESTAMP=`date -u +%Y-%m-%dT%H:%M+0000 2>/dev/null || date`
+      else
+        BUILD_TIMESTAMP="$enableval"
+      fi],
+     [BUILD_TIMESTAMP="<none>"])
 AC_SUBST(BUILD_TIMESTAMP)
 AC_DEFINE_UNQUOTED(BUILD_TIMESTAMP, "$BUILD_TIMESTAMP",
                    [The time this package was configured for a build])
 
+
 #
 # Options to disable some regression tests
 #