Workaround for gcc<4.6 cpp bug

Old g++ preprocessors don't report __cplusplus correctly, so let's
resort to ANTIOCH_HAVE_CXX11 to figure out whether we can use decltype
or not.  This helps to fix a hundred false positive warnings about
signed-unsigned-comparison in assertions when using older gcc.
Enable C++11 by default

Even when we're not using VexCL, C++11 support is nice to use for the
ability to use our decltype-enabled assertions and avoid overzealous
compiler warnings.