This replaces Automake's `build-aux/mdate-sh' with our own
`build-aux/mdate-from-git.scm' to use reproducible timestamps from Git
instead.
* build-aux/mdate-from-git.scm: New script.
* bootstrap: Use it to replace build-aux/mdate-sh.
* Makefile.am (EXTRA_DIST): Add it.
Change-Id: I17d0a7de9ffea397129c0db1728f86e28a4e245f