Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline
appears in a // comment. (Both forms have the same effect.)
Turns on all optional warnings which are desirable for normal code. At present this is -Wcomment,
-Wtrigraphs, -Wmultichar and a warning about integer promotion causing a change of sign in "#if"
expressions. Note that many of the preprocessor's warnings are on by default and have no options to