Log Message:
Update to gmp-4.2.3.

Changes between GMP version 4.2.2 and 4.2.3:

  Bugs:
  * Fix x86 CPU recognition code to properly identify recent AMD and Intel
    64-bit processors.
  * The >> operator of the C++ wrapper gmpxx.h now does floor rounding, not
    truncation.
  * Inline semantics now follow the C99 standard, and works with recent GCC
    releases.
  * C++ bitwise logical operations work for more types.
  * For C++, gmp.h now includes cstdio, improving compiler compatibility.
  * Bases > 36 now work properly in mpf_set_str.

  Speedups:
  * None, except that proper processor recognition helps affected processors.

  Features:
  * The allocation functions now detect overflow of the mpz_t type.  This means
    that overflow will now cause an abort, except when the allocation
    computation itself overflows.  (Such overflow can probably only happen in
    powering functions; we will detect powering overflow in the future.)