Fixed build breakage from 37e946683f1bb6a.

In 37e946683f1bb6a I fixed an issue where the build fails due to a
warning, which in GCC >=4.4 seems to work correctly. In older versions
of GCC such as 4.2 and 4.1, the unknown flag isn't handled gracefully
(GCC returns an error). This must have been added in a later version of
GCC. The feature is documented here:

http://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html