po/Makefile.am: use SOURCE_DATE_EPOCH to set POT creation date
po/Makefile.am: use SOURCE_DATE_EPOCH to set POT creation date

When building RPMs for libvirt the PO files are part of libvirt-libs
package.  Now that we generate libvirt.pot during build time the POT
creation date is also generated at that time.

The issue here is that when building libvirt-libs for x86_64 and i686
the generated libvirt.pot file will have different POT creation date
which affects installed PO files as well which leads to conflict when
installing both x86_64 and i686 packages.

Signed-off-by: Pavel Hrdina <d4772d05997b8abf035041e3b4f4996380ea7e7a@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

zippy2/libvirt,fabianfreyer/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,crobinso/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,libvirt/libvirt,jfehlig/libvirt,jardasgit/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,crobinso/libvirt,olafhering/libvirt,nertpinx/libvirt,zippy2/libvirt