stdlib/gmp-impl.h: Silence -Wundef warning for USE_STACK_ALLOC

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.