The easy part is to add NIX_CFLAGS_COMPILE for "regular" libraries.
A bit more tricky is to add the required flags for libclang to find
libstdcxx. For this we parse arguments to bindgen to look for
-x c++ or -xc++ and if found add NIX_CXXSTDLIB_COMPILE to the arguments.
This variable is populated by a complex dance of setupHooks. We trigger
this by adding clang to propagatedBuildInputs. A more subtle way may
exist.
