Build: spec: rationalize pre-release vs. tags logic

• spec file should silently ignore "--with pre_release" condition when commit variable carries the tag alias instead of the hash; note that if the tag itself implies pre-release with -rcX suffix, pre-release logic is selected regardless

• similarly, GNUMakefile should not push the version to be substituted in the spec file up when the current checkout happens to be tagged

Disclaimer: this is to solve rough edges in COPR integration, because
currently a build from 1.1 branch (i.e. enforcing "--with pre_release"),
HEAD resolving to Pacemaker-1.1.16 tag, the generated version would be
"1.1.17-0.1.1.1.16.git" instead of expected "1.1.16-1".

To some extent, the whole arrangement relies on established release
procedures, e.g., in terms of restoring work on release ("decimal dot
decimal") branch at the same time updated version.m4 is pushed there.