spec: Add support for user-flags configure option

Passing -ggdb3 (or -g3) during compiler may result in corrupted
debuginfo files (bug in debugedit - for Fedora filed as a
https://bugzilla.redhat.com/show_bug.cgi?id=1708786). Until the bug is
fixed it's possible to ether change configure to add -ggdb2/-g2 or use
already existing --enable-user-flags option and rely on environment set
by rpmbuild.

Patch implements second option so RPM distros without broken debugedit
are not affected.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>