macos: suppress some very chatty warnings

We were seeing thousands of warnings in the macOS SDK when building on
LLVM 10. Probably due to new warnings in that toolchain.

Let's suppress them and make build output reasonable again.

cpython-unix/build-cpython.sh

@@ -113,6 +113,10 @@ if [ "${PYBUILD_PLATFORM}" = "macos" ]; then
 
     # Prevent using symbols not supported by current macOS SDK target.
     CFLAGS="${CFLAGS} -Werror=unguarded-availability-new"
+
+    # Suppress extremely frequent warnings we see in macOS SDK headers
+    # with LLVM 10.
+    CFLAGS="${CFLAGS} -Wno-nullability-completeness -Wno-expansion-to-defined"
 fi
 
 CPPFLAGS=$CFLAGS