[Bug jit/64780] toplevel configure should reject jit as a language if --enable-host-shared is not supplied

["dmalcolm at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64780

--- Comment #1 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
We seem to have two choices here:

(A) default to --enable-host-shared when jit is an enabled language
(B) have the toplevel configure reject jit as language if --enable-host-shared
is not supplied.

FWIW apparently Darwin defaults to position-independent code, so it's not
explicitly needed there.

That said, I think I prefer (B), since there is a performance cost: my thinking
here is that there are people who perform benchmarking of GCC (and publish
their results on prominent websites).  If they turn on the jit and use the same
configuration to do their benchmarking of the rest of GCC, they'll see GCC 5 be
apparently slower than earlier releases.  You're a gcc expert, and you ran into
this issue, so it's clearly unreasonable to expect a 3rd-party reviewer to
notice this detail on their own.

So I'm thinking of implementing (B), and putting in a note in the error message
recommending that people configure and build GCC twice to avoid the performance
hit, so that it can be self-documenting.