UseSWIG: Change the library suffix to 'dylib' for C# on macOS

When testing on macOS, the command `swig_add_library("mytarget")`
creates a library `libmytarget.so` instead of `libmytarget.dylib`.
Subsequently, the library is not found by `dotnet` and the
swig-generated `DllImport` statements.

Fixes: #23967

Modules/UseSWIG.cmake

@@ -990,6 +990,9 @@ function(SWIG_ADD_LIBRARY name)
       endif()
       set_target_properties (${target_name} PROPERTIES PREFIX "")
     endif()
+    if (APPLE)
+      set_target_properties (${target_name} PROPERTIES SUFFIX ".dylib")
+    endif ()
   else()
     # assume empty prefix because we expect the module to be dynamically loaded
     set_target_properties (${target_name} PROPERTIES PREFIX "")