We build with -nostdinc++ and add our own header path via
LIBCXXABI_LIBCXX_INCLUDES. However cmake tries to be clever and if
LIBCXXABI_LIBCXX_INCLUDES happens to match the compilers system path
it will remove the -I flag meaning we can't access any C++ headers.
This problem only happens when LIBCXXABI_LIBCXX_INCLUDES and the
compilers default C++ include path are the same thing. This happens on the
wasm waterfall where we build and install libcxx into the compiler default
location before we build libcxxabi.
Ideally cmake would be able see that we are using -nostdinc++ and
disable this behaviour.