Missing glibc 2.6 prereq for HAVE_ATOMIC dependency

We have an environment where GCC > 4.0.1 (4.1.2) but glibc < 2.6 (2.5) (I know, weird).  Thus linking would fail because even though HAVE_ATOMIC was set to true, there is no 'sync_add_and_fetch' .  The simple one-line change is to also require glibc 2.6.