Tweak portability.h for uClibc version in buildroot defconfig.

Tweak portability.h for uClibc version in buildroot defconfig.

I've been locally patching uClibc to not violate posix-2008 (you don't need
to define a GNU macro to get a posix function), but uClibc is obsolete and
moribund (development peaked in 2006, last bugfix release was over 2 years
ago), and the largest remaining user (buildroot) doesn't bother to apply such
a patch. Since even buildroot is slowly migrating to musl-libc, just do the
portability tweak for what the last release of the old thing actually did.