Conditionalize compiler flags that are specific to clang to fix errors
of the following forms on GCC-based architectures:
cc1plus: error: unrecognized command line option "-Wno-c++11-narrowing" cc1plus: error: unrecognized command line option "-Wno-reserved-user-defined-literal" cc1plus: warning: unrecognized command line option '-Wno-error-narrowing'
Approved by: portmgr (tier-2 blanket)