CheckPrototypeDefinition: Avoid if() auto-dereference

Convert from the old if(MATCHES) hack to if(DEFINED) to check whether
the result variable is already set.

Modules/CheckPrototypeDefinition.cmake

@@ -57,7 +57,7 @@ get_filename_component(__check_proto_def_dir "${CMAKE_CURRENT_LIST_FILE}" PATH)
 
 function(CHECK_PROTOTYPE_DEFINITION _FUNCTION _PROTOTYPE _RETURN _HEADER _VARIABLE)
 
-  if ("${_VARIABLE}" MATCHES "^${_VARIABLE}$")
+  if (NOT DEFINED ${_VARIABLE})
     set(CHECK_PROTOTYPE_DEFINITION_CONTENT "/* */\n")
 
     set(CHECK_PROTOTYPE_DEFINITION_FLAGS ${CMAKE_REQUIRED_FLAGS})