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

cmake/modules/DuneAddPybind11Module.cmake

@@ -72,6 +72,13 @@ 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
+  if (APPLE)
+    set_target_properties(${PYBIND11_MODULE_NAME} PROPERTIES SUFFIX ".so")
+  endif()
+
   target_compile_definitions(${PYBIND11_MODULE_NAME} PRIVATE ${PYBIND11_MODULE_COMPILE_DEFINITIONS})
   dune_target_enable_all_packages(${PYBIND11_MODULE_NAME})