[!799] [cmake] Force ".so" suffix for pybind11 module libraries on macOS

Merge branch 'feature/python-force-library-so-suffix-for-macos' into 'master'

ref:core/dune-common\> The issue is discussed in this old thread on
stackoverflow:
[https://stackoverflow.com/questions/2488016/how-to-make-python-load-dylib-on-osx]

Somehow Python refuses to import dynamic libraries with the suffix .dylib (I
get an import error) which is the common suffix for shared libs in macOS.
Telling CMake to use .so instead as suggested in the stackoverflow thread
works for me.

The Python libs on for my Python 3.7 from homebrew all have .so endings just
like in the stackoverflow answer.

Fixes [#201]

See merge request [!799]

  [https://stackoverflow.com/questions/2488016/how-to-make-python-load-dylib-on-osx]:
    https://stackoverflow.com/questions/2488016/how-to-make-python-load-dylib-on-osx
  [#201]: gitlab.dune-project.org/NoneNone/issues/201
  [!799]: gitlab.dune-project.org/core/dune-common/merge_requests/799


Closes #201

cmake/modules/DuneAddPybind11Module.cmake

@@ -72,6 +72,17 @@ function(dune_add_pybind11_module)
 
   add_library(${PYBIND11_MODULE_NAME} SHARED ${PYBIND11_MODULE_SOURCES})
   set_target_properties(${PYBIND11_MODULE_NAME} PROPERTIES PREFIX "")
+
+  # force '.so' as library suffix on macOS due to a problem in Python
+  # https://stackoverflow.com/questions/2488016/how-to-make-python-load-dylib-on-osx
+  # and add -undefined dynamic_lookup flag to linker
+  # https://pybind11.readthedocs.io/en/stable/compiling.html#building-manually
+  if (APPLE)
+    set_target_properties(${PYBIND11_MODULE_NAME} PROPERTIES SUFFIX ".so")
+    target_link_options(${PYBIND11_MODULE_NAME} PRIVATE "-undefined")
+    target_link_options(${PYBIND11_MODULE_NAME} PRIVATE "dynamic_lookup")
+  endif()
+
   target_compile_definitions(${PYBIND11_MODULE_NAME} PRIVATE ${PYBIND11_MODULE_COMPILE_DEFINITIONS})
   dune_target_enable_all_packages(${PYBIND11_MODULE_NAME})