This starts by setting the gnucash version number in the 'project' call.
This will result in a number of variables set by cmake. The remainder
of this commit is to reuse the auto-generated
PROJECT_NAME, PROJECT_VERSION, PROJECT_VERSION_MAJOR
PROJECT_VERSION_MINOR, PROJECT_VERSION_PATCH and PROJECT_VERSION_TWEAK
instead of all the various instances of PACKAGE, GNUCASH_MAJOR_VERSION
and so on.
One parameter is worth mentioning - GETTEXT_PACKAGE
GnuCash is not using this directly (any more) but it has to be set before
including gi18n-lib.h.
