Hardcoded value of GMP_LIMB_BITS impedes bi-arch builds

[Jan Engelhardt]

Version: gmp-6.2.0

When ./configure has run, the GMP_LIMB_BITS define in gmp.h is hardcoded 
to 32 or 64, depending on arch. This makes it impossible to use that 
header file for a x86_64-driven build with gcc -m32.
It is my (naive) view that declaring

	#define GMP_LIMB_BITS (sizeof(mp_limb_t) * CHAR_BIT)

could address the issue in short order, though, upon last minute 
searching, I find this problem to have been previously reported in 
https://gmplib.org/list-archives/gmp-bugs/2012-March/002593.html with no 
action taken.