FindPythonLibs: unset temporary _PREFIX variable

To avoid pollution, unset variables that are only meant for local use.

Modules/FindPythonLibs.cmake

@@ -53,16 +53,18 @@
 set(_Python_LIBRARY_PATH_HINT)
 if(PYTHON_EXECUTABLE)
   if(WIN32)
-    get_filename_component(_PREFIX ${PYTHON_EXECUTABLE} PATH)
-    if(_PREFIX)
-      set(_Python_LIBRARY_PATH_HINT ${_PREFIX}/libs)
+    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(_PREFIX ${PYTHON_EXECUTABLE} PATH)
-    get_filename_component(_PREFIX ${_PREFIX} PATH)
-    if(_PREFIX)
-      set(_Python_LIBRARY_PATH_HINT ${_PREFIX}/lib)
+    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()
+    unset(_Python_PREFIX)
   endif()
 endif()
 
@@ -175,11 +177,12 @@ foreach(_CURRENT_VERSION ${_Python_VERSIONS})
 
     # Use the library's install prefix as a hint
     set(_Python_INCLUDE_PATH_HINT)
-    get_filename_component(_PREFIX ${PYTHON_LIBRARY} PATH)
-    get_filename_component(_PREFIX ${_PREFIX} PATH)
-    if(_PREFIX)
-      set(_Python_INCLUDE_PATH_HINT ${_PREFIX}/include)
+    get_filename_component(_Python_PREFIX ${PYTHON_LIBRARY} PATH)
+    get_filename_component(_Python_PREFIX ${_Python_PREFIX} PATH)
+    if(_Python_PREFIX)
+      set(_Python_INCLUDE_PATH_HINT ${_Python_PREFIX}/include)
     endif()
+    unset(_Python_PREFIX)
 
     # Add framework directories to the search paths
     set(PYTHON_FRAMEWORK_INCLUDES)