GMP and clang bugginess

[Torbjörn Granlund]

GMP triggers bugs in clang on every platform where we tried this
compiler.  Some configs work, though.

To see how bad it is, please take a look here:

  https://gmplib.org/devel/tm-date.html

I think we would help our users by making it hard to use clang with the
next release.  What do you think?

My idea is to reject clang with an error message along these lines:

error: clang is not able to correctly compile GMP.  To override, use
--enable-clang.  Make sure to run "make check" and don't use the library
of this fails.


-- 
Torbjörn
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