The upstream version of GMP has long removed this conditional
altogether in this commit:
changeset: 5254:88618a4694ac
user: Kevin Ryde <user42@zip.com.au>
date: Sun Jun 17 01:37:27 2001 +0200
So just turn the #if into an #ifdef to silence the warning.
ChangeLog:
2014-05-14 Will Newton <will.newton@linaro.org>
* stdlib/gmp-impl.h: Test USE_STACK_ALLOC #ifdef
rather than #if.