std=c++11 flag in client CMakeFiles.txt causes error

Created by: iwrinc

The error is at https://github.com/DynamoRIO/dynamorio/blob/master/make/DynamoRIOConfig.cmake.in :354 Escaping ++ in cur_flags and flag vars solves this issue:

352: get_source_file_property(cur_flags ${src} COMPILE_FLAGS)
++    string(REPLACE "++" "\\+\\+" cur_flags_esc ${cur_flags})
         foreach (flag ${tgt_flags})
++       string(REPLACE "++" "\\+\\+" flag_esc ${flag})
           if (NOT cur_flags_esc MATCHES " ${flag_esc}")
              _DR_append_property_string(SOURCE ${src} COMPILE_FLAGS "${flag}")
           endif ()