CheckIPOSupported: Simplify result reporting logic

Simplify the test for whether a `RESULT` argument was given to the
`check_ipo_supported` call.  Also do not set an empty variable name
if `OUTPUT` was not given.

Modules/CheckIPOSupported.cmake

@@ -56,13 +56,14 @@ include(CMakeParseArguments) # cmake_parse_arguments
 # X_RESULT - name of the final result variable
 # X_OUTPUT - name of the variable with information about error
 macro(_ipo_not_supported output)
-  string(COMPARE EQUAL "${X_RESULT}" "" is_empty)
-  if(is_empty)
+  if(NOT X_RESULT)
     message(FATAL_ERROR "IPO is not supported (${output}).")
   endif()
 
   set("${X_RESULT}" NO PARENT_SCOPE)
-  set("${X_OUTPUT}" "${output}" PARENT_SCOPE)
+  if(X_OUTPUT)
+    set("${X_OUTPUT}" "${output}" PARENT_SCOPE)
+  endif()
 endmacro()
 
 # Run IPO/LTO test