GenerateExportHeader: Fix add_compiler_export_flags regression

The mechanical conversion in commit 5d0d980d (Use string(APPEND) in
Modules, 2016-07-28) accidentally introduced use of

  string(APPEND ... PARENT_SCOPE)

Split that into the string(APPEND) and set(PARENT_SCOPE) pieces.

Modules/GenerateExportHeader.cmake

@@ -415,6 +415,7 @@ function(add_compiler_export_flags)
   if(ARGC GREATER 0)
     set(${ARGV0} "${EXTRA_FLAGS}" PARENT_SCOPE)
   else()
-    string(APPEND CMAKE_CXX_FLAGS " ${EXTRA_FLAGS}" PARENT_SCOPE)
+    string(APPEND CMAKE_CXX_FLAGS " ${EXTRA_FLAGS}")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}" PARENT_SCOPE)
   endif()
 endfunction()