Break the toolchain assumption of LLVM on Prefix/macOS.

At Prefix/macOS, the system compiler is GCC with libstdc++.

Reference: https://github.com/pybind/pybind11/pull/4639

Signed-off-by: Benda Xu <heroxbd@gentoo.org>

Index: pybind11-2.10.4/pybind11/setup_helpers.py
===================================================================
--- pybind11-2.10.4.orig/pybind11/setup_helpers.py
+++ pybind11-2.10.4/pybind11/setup_helpers.py
@@ -154,9 +154,6 @@ class Pybind11Extension(_Extension):  #
             c_cpp_flags = shlex.split(env_cflags) + shlex.split(env_cppflags)
             if not any(opt.startswith("-g") for opt in c_cpp_flags):
                 cflags += ["-g0"]
-            if MACOS:
-                cflags += ["-stdlib=libc++"]
-                ldflags += ["-stdlib=libc++"]
         self._add_cflags(cflags)
         self._add_ldflags(ldflags)