Merge branch 'Macos_lib_extension' into 'Release_1.9.4'
Macos lib extension -> force .so CMakeLists.txt files of userfct and symbolic are modified to force the extension of the lib to be `.so` rather than `.dylib` when compiling with FLAG_SEPARATE_BUILD to ON. This is done for MacOS only. This avoids to modify the function that load the dynamic library at runtime. See merge request !90