* c-decl.c: Add warn_multichar.

        (c_decode_option): Handle -Wno-multichar.
        * c-lex.c (yylex): Check it.
        * c-tree.h: Declare it.
        * toplev.c (lang_options): Add it.
        * invoke.texi: Document it.


git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@20968 138bc75d-0d04-0410-961f-82ee72b054a4