Fix copied from libvirt, commit by Eric Blake.
glibc 2.15 (on Fedora 17) coupled with explicit disabling of
optimization during development dies a painful death:
/usr/include/features.h:314:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
Work around this by only conditionally defining _FORTIFY_SOURCE,
in the case where glibc can actually use it. The trick is using
AH_VERBATIM instead of AC_DEFINE.