Including <limits.h> in gmp-impl.h

[Torbjorn Granlund]

Vincent Lefevre <vincent at vinc17.net> writes:

  Note that if you want to care of non-two's-complement
  implementations, you should write:
  
  #define __GMP_USHRT_MAX  (0 + (unsigned short) -1)
  
Is that out-of-range conversion really Proper C?

Torbjörn
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