Update CMake boost python logic

We need to have at least one other required library, otherwise FindBoost will consider itself successful even if it didn't find anything.
Also, add Python 3.8 support by using the Python version numbers as library version numbers.

CMakeLists.txt

@@ -652,9 +652,14 @@ else()
   list ( APPEND waLBerla_OPTIONAL_BOOST_COMPONENTS system )
 endif()
 
-if ( WALBERLA_BUILD_WITH_PYTHON AND WALBERLA_CXX_COMPILER_IS_MSVC )
+if ( WALBERLA_BUILD_WITH_PYTHON )
+  if( WALBERLA_CXX_COMPILER_IS_MSVC )
     get_filename_component(PYTHON_REQUIRED_LIB ${PYTHON_LIBRARY} NAME_WE)
     list( APPEND waLBerla_REQUIRED_BOOST_COMPONENTS ${PYTHON_REQUIRED_LIB} )
+  elseif( WALBERLA_USE_STD_FILESYSTEM OR WALBERLA_USE_STD_EXPERIMENTAL_FILESYSTEM )
+    list( APPEND waLBerla_REQUIRED_BOOST_COMPONENTS system )
+    list( REMOVE_ITEM waLBerla_OPTIONAL_BOOST_COMPONENTS system )
+  endif()
 endif()
 
 # This variable is necessary, if the CMAKE version used is not aware of a more recent boost version (keep this up to date!)
@@ -749,11 +754,13 @@ if ( WALBERLA_BUILD_WITH_PYTHON AND NOT WALBERLA_CXX_COMPILER_IS_MSVC)
         SET(_boost_MULTITHREADED "-mt")
     endif()
     if( PYTHON_LIBRARY MATCHES "python3" )
+        string(REPLACE "." ";" VERSION_LIST ${PYTHONLIBS_VERSION_STRING})
+        list(GET VERSION_LIST 0 PY_VER_MAJOR)
+        list(GET VERSION_LIST 1 PY_VER_MINOR)
         find_library( BOOST_PYTHON_LIBRARY NAMES
-                boost_python37${_boost_MULTITHREADED} boost_python36${_boost_MULTITHREADED} boost_python35${_boost_MULTITHREADED}
-                boost_python-py37${_boost_MULTITHREADED} boost_python-py36${_boost_MULTITHREADED} boost_python-py35${_boost_MULTITHREADED}
-                boost_python-py34${_boost_MULTITHREADED} boost_python-py33${_boost_MULTITHREADED}
-                boost_python3${_boost_MULTITHREADED}
+                boost_python${PY_VER_MAJOR}${PY_VER_MINOR}${_boost_MULTITHREADED}
+                boost_python-py${PY_VER_MAJOR}${PY_VER_MINOR}${_boost_MULTITHREADED}
+                boost_python${PY_VER_MAJOR}${_boost_MULTITHREADED}
                 boost_python${_boost_MULTITHREADED}
                 PATHS ${Boost_LIBRARY_DIRS} NO_DEFAULT_PATH )
     else()