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[Torbjörn Granlund]

GMP 6 has issues with FreeBSD, which we do not plan to fix.  The full
list is under MISC at <https://gmplib.org/gmp6.0.html>.

The main issue is the broken m4.  A plain 'configure' in gmp with fail
unless a working (read GNU) m4 is available.  NetBSD before 6.0 has the
same buggy m4.

But then 32-bit clang miscompiles gmp.  There is no workaround for that,
and we'll end up in a circular dependency loop where gmp needs gcc and
gcc needs gmp.  (I don't know which gmp version are affected by this.)

The conservative choice for gcc is to prefer gmp 5.1.3.


Torbjörn
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