This effectively replaces the use of GNUCASH_BUILD_DATE with GNUCASH_SCM_REV_DATE.
The latter is extracted from the current commit if building from some kind of vcs
(currently only works correctly for svn and git). The info extracted while building
from vcs is then also added to the dist tarball so it's available when building
from tarball as well (via the file libgnucash/core-utils/gnc-vcs-info.h).
The same date is also used to set the date in gnucash' man page document.
A practical detail: I have changed the substitution variables in the man page template
from @- -@ to ${} so we could leverage CONFIGURE_FILE in cmake. The necessary
related adjustments have also been made to Makefile.am's substitution rules.
