cmake: use modern alternative for get_filename_component()

- use `cmake_path()` to query filenames, with CMake 3.20 or upper.
  https://cmake.org/cmake/help/v4.1/command/cmake_path.html#query

- also quote the value passed to `get_filename_component()` where
  missing. (Could not cause an actual issue as used in the code.)

Closes libssh2#1673

Author: vszakats

src/CMakeLists.txt

@@ -291,8 +291,13 @@ if(NOT LIBSSH2_DISABLE_INSTALL)
         if(_lib MATCHES "/")
           # This gets a bit more complex, because we want to specify the
           # directory separately, and only once per directory
-          get_filename_component(_libdir ${_lib} DIRECTORY)
-          get_filename_component(_libname ${_lib} NAME_WE)
+          if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.20)
+            cmake_path(GET _lib PARENT_PATH _libdir)
+            cmake_path(GET _lib STEM _libname)
+          else()
+            get_filename_component(_libdir "${_lib}" DIRECTORY)
+            get_filename_component(_libname "${_lib}" NAME_WE)
+          endif()
           if(_libname MATCHES "^lib")
             if(CMAKE_VERSION VERSION_GREATER_EQUAL 3.20)
               cmake_path(SET _libdir NORMALIZE "${_libdir}")