Fix make_etags failure on Mac.

Fix make_etags failure on Mac.

Previously make_etags always ran make_ctags -e when make_etags was
executed.  However, because non-Exuberant ctags on Mac does not
support -e option (and also on other platforms including old Linux),
ctags failed. To avoid the failure change make_ctags so that if
non-Exuberant ctags is used and ctags -e option is requested, run
etags command instead. If etags command does not exist, make_ctags
will fail.

Also refactor make_ctags and tweak make_etags to emit proper usage
message.

Author: Fujii Masao
Reviewed-by: Tatsuo Ishii
Discussion: https://www.postgresql.org/message-id/369c13b9-8b0f-d6f9-58fc-61258ec8f713%40oss.nttdata.com