The C++ bits of gegl-parallel.h require C++11 (they use lambdas);
avoid including them for older versions of C++.

We already condition our own C++ compilation on the availability of
C++14, but since gegl-parallel.h is a public header, it should be
possible to include it (even if not use its C++ interface) in
older C++ versions.  We *could* make the C++ interface compatible
with older versions, but laziness...