Define MPFR_USE_INTMAX_T so that mpfr.h assumes intmax_t is available.

Define MPFR_USE_INTMAX_T so that mpfr.h assumes intmax_t is available.

mpfr.h uses a non-portable test to guess if intmax_t is available and
if API functions using intmax_t should be exposed.  Define
MPFR_USE_INTMAX_T to override the non-portable test and always expose
these functions.  This fixes the build on platforms where the test
guesses incorrectly.

gdb/ChangeLog:

	* target-float.c [HAVE_LIBMPFR]: Define MPFR_USE_INTMAX_T.