Outdated libtool creates trouble on macOS

[Torbjörn Granlund]

FX Coudert <fxcoudert at gmail.com> writes:

  PS: second patchlet, to deal with these two macros:

  > warning: The macro `AC_HEADER_STDC' is obsolete.
  > warning: The macro `AC_HEADER_TIME' is obsolete.

If you, Marc, can take a look at those proposed changes, I think they
should be applied to the head gmp repo, not the gmp-6.3 one.

Currently and for another week, the nightly builds whack gmp-6.3, then
we will switch to the head repo.  That will then be a broad test of any
configure changes.

(After that, we can consider merging the changes to gmp-6.3 for inclusion
in a 6.3.1.)

-- 
Torbjörn
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