Hi
MCUXpresso IDE compiler has -I (include paths), -include(include files). Linker has -l (include libs), -L(include search path). This is sufficient to find system files. Could you please let me know why you need -isystem?
Seems MCUXpresso IDE doesn't arrange a place for -isystem . If you insist adding it, you can add it in other flag:

Hope this helps,
Jun Zhang