issue with Microsoft compiler

[Paul Zimmermann]

       Hi,

someone reported to me the following issue:

   Microsoft (R) C/C++ Optimizing Compiler Version 19.34.31937 for x64

   will not compile line 2230 in gmp.h:

      *__gmp_rp = (- *__gmp_up) & GMP_NUMB_MASK;

   giving error C4146: unary minus operator applied to unsigned type, 
   result still unsigned.

Just in case this warning can be avoided.

Best regards,
Paul