Better context for "TAP tests not enabled" error message

I got the error message in the subject and was unsure how to continue; I
didn't see any hits for the error message on the mailing list, and it was
hard to determine from the context around the error in the
Makefile.global.in about the best way to solve the problem.

This patch amends the error message to give help to the user.

I ran "make check" at the top level with this patch enabled and 209 tests
passed. I also ran "make check" in src/test/ssl without TAP enabled and
verified that I got the new error message. I also verified that compiling
with --enable-tap-tests fixes the error in question.

This patch does not include regression tests.

Another way to fix this issue could be to put the exact text of the error
message in the documentation or the wiki, with instructions on how to fix
it - the first thing I did was punch the error message into Google, if a
match for the error message came up with instructions on how to fix it,
that would also help.

This is the first patch that I've submitted to Postgres, I believe that
I've followed the guidelines on the patch submission page, but please let
me know if I missed anything.

Kevin

--
Kevin Burke
phone: 925-271-7005 | kevin.burke.dev