Avoid 'unexpanded macro' warning in mock srpm builds

When preparing the srpm in mock or other minimal environments, the use
of %{_emacs_version} causes a spurious warning:

  Possible unexpanded macro in: Requires: emacs-filesystem >= %{_emacs_version}

Prevent the warning with a check that the macro is defined before use.

(There is another use of %{_emacs_version}, but it only applies to EL-6
where the warning is not present.  Just ignore it.)

git.spec

@@ -221,7 +221,7 @@ Requires:       perl(Term::ReadKey)
 %endif # ! defined perl_bootstrap
 Requires:       perl-Git = %{version}-%{release}
 
-%if %{emacs_filesystem}
+%if %{emacs_filesystem} && %{defined _emacs_version}
 Requires:       emacs-filesystem >= %{_emacs_version}
 %endif # emacs_filesystem