Change macOS handling of undefined symbols

Newer version of XCode deprecate the ``-undefined dynamic_lookup`` flag
that pybind11 has used to avoid linking to a specific Python shared
library. Every use of it now lead to the following noisy warning
message.
```
ld: warning: -undefined dynamic_lookup may not work with chained fixups
```
More details on this are available here:
https://mjtsai.com/blog/2021/06/30/faster-app-launching-in-ios-15-and-monterey/

While this seems somewhat specific to Swift, it is problematic if a core
linker feature that pybind11 depends on is declared unreliable.

I have been able to replace these flags with
```
-undefined suppress -flat_namespace
```

I am not sure if this suits everyone. The purpose of the PR is to raise
the issue and starts a discussion on what to do.

Author: wjakob

docs/compiling.rst

@@ -583,12 +583,12 @@ using ``pip`` or ``conda``. If it hasn't, you can also manually specify
 ``python3-config --includes``.
 
 On macOS: the build command is almost the same but it also requires passing
-the ``-undefined dynamic_lookup`` flag so as to ignore missing symbols when
+the ``-undefined suppress -flat_namespace`` flag so as to ignore missing symbols when
 building the module:
 
 .. code-block:: bash
 
-    $ c++ -O3 -Wall -shared -std=c++11 -undefined dynamic_lookup $(python3 -m pybind11 --includes) example.cpp -o example$(python3-config --extension-suffix)
+    $ c++ -O3 -Wall -shared -std=c++11 -undefined suppress -flat_namespace $(python3 -m pybind11 --includes) example.cpp -o example$(python3-config --extension-suffix)
 
 In general, it is advisable to include several additional build parameters
 that can considerably reduce the size of the created binary. Refer to section

tools/pybind11Common.cmake

@@ -75,15 +75,15 @@ if(CMAKE_VERSION VERSION_LESS 3.13)
   set_property(
     TARGET pybind11::python_link_helper
     APPEND
-    PROPERTY INTERFACE_LINK_LIBRARIES "$<$<PLATFORM_ID:Darwin>:-undefined dynamic_lookup>")
+    PROPERTY INTERFACE_LINK_LIBRARIES "$<$<PLATFORM_ID:Darwin>:-undefined suppress -flat_namespace>")
 else()
   # link_options was added in 3.13+
   # This is safer, because you are ensured the deduplication pass in CMake will not consider
   # these separate and remove one but not the other.
   set_property(
     TARGET pybind11::python_link_helper
     APPEND
-    PROPERTY INTERFACE_LINK_OPTIONS "$<$<PLATFORM_ID:Darwin>:LINKER:-undefined,dynamic_lookup>")
+    PROPERTY INTERFACE_LINK_OPTIONS "$<$<PLATFORM_ID:Darwin>:LINKER:-undefined,suppress,-flat_namespace>")
 endif()
 
 # ------------------------ Windows extras -------------------------