FindPythonLibs: Tolerate a non-absolute PYTHON_EXECUTABLE

The `PYTHON_EXECUTABLE` variable normally contains an absolute path, but
tolerate cases when it does not without calling `get_filename_component`
with an incorrect number of arguments.

Closes: #16452

Modules/FindPythonLibs.cmake

@@ -42,16 +42,16 @@
 
 # Use the executable's path as a hint
 set(_Python_LIBRARY_PATH_HINT)
-if(PYTHON_EXECUTABLE)
+if(IS_ABSOLUTE "${PYTHON_EXECUTABLE}")
   if(WIN32)
-    get_filename_component(_Python_PREFIX ${PYTHON_EXECUTABLE} PATH)
+    get_filename_component(_Python_PREFIX "${PYTHON_EXECUTABLE}" PATH)
     if(_Python_PREFIX)
       set(_Python_LIBRARY_PATH_HINT ${_Python_PREFIX}/libs)
     endif()
     unset(_Python_PREFIX)
   else()
-    get_filename_component(_Python_PREFIX ${PYTHON_EXECUTABLE} PATH)
-    get_filename_component(_Python_PREFIX ${_Python_PREFIX} PATH)
+    get_filename_component(_Python_PREFIX "${PYTHON_EXECUTABLE}" PATH)
+    get_filename_component(_Python_PREFIX "${_Python_PREFIX}" PATH)
     if(_Python_PREFIX)
       set(_Python_LIBRARY_PATH_HINT ${_Python_PREFIX}/lib)
     endif()