bug in t-perfpow for mingw64

[Alexander Maly]

>
> Please try the patch from the status section of the GMP webpage:
> http://gmplib.org:8000/gmp-5.0/raw-rev/794410151f5f
>
> --
> Torbjörn

Works for me. Though I did it manually, not by the program 'patch',
since I have already reformatted text of t-perfpow.c when discovered
why it was not working. But there is a subtle moment: in line

gmp_printf("n = %Zu\nprimes[0] = %Zu\nexp[0] = %lu\n", n, primes[0], exp[0]);
it would print exp[0] as unsigned long, and I do not know what is the
format for mp_limb_t in gmp_printf.
So may be it is better to leave exp and e variables as unsigned longs?
They should have very small values,
and their addresses are never else used, so it should not make big difference.
Strange that when I turned warnings on by "-Wall -W" the compiler did
not tell me about wrong arguments size.
However, assigning such attributes to gmp_printf arguments is very
compiler-specific, and may be just impossible if the format string has
non-standart symbols, but it might increase type-safety.
Another offtopic question: when I turned warnings on, the compiler
written the following similar warnings:
In file included from t-perfpow.c:26:0:
../../gmp-impl.h: In function 'mpn_toom32_mul_itch':
../../gmp-impl.h:4568:63: warning: signed and unsigned type in
conditional expression
../../gmp-impl.h: In function 'mpn_toom43_mul_itch':
../../gmp-impl.h:4584:55: warning: signed and unsigned type in
conditional expression
../../gmp-impl.h: In function 'mpn_toom52_mul_itch':
../../gmp-impl.h:4592:63: warning: signed and unsigned type in
conditional expression
../../gmp-impl.h: In function 'mpn_toom62_mul_itch':
../../gmp-impl.h:4606:59: warning: signed and unsigned type in
conditional expression

They are because in that lines the divisors (small constants, like 3)
were casted to (size_t), that is unsigned, while all the other
variables are signed. Is that for a reason, or they may be casted to
(ssize_t) as well (ssize_t is similiar to size_t, but signed) or
leaved as is? I am asking that because the compiler is very helpful in
finding neat errors if warnings are properly turned on and the found
warnings are cleaned in code.