Suppress warnings generated by -Wwrite-strings

[kou]

Here are suppressed warnings:

src/fmt_fp.c: In function 'fmt_fp':
src/fmt_fp.c:124:16: warning: initialization discards 'const' qualifier from pointer target type
     char *ss = (t&32)?"inf":"INF";
                ^
src/fmt_fp.c:125:17: warning: assignment discards 'const' qualifier from pointer target type
     if (y!=y) ss=(t&32)?"nan":"NAN";
                 ^
mrbgems/mruby-string-ext/src/string.c: In function 'mrb_str_succ_bang':
mrbgems/mruby-string-ext/src/string.c:302:27: warning: assignment discards 'const' qualifier from pointer target type
       if (e == b) prepend = "1";
                           ^
mrbgems/mruby-string-ext/src/string.c:305:27: warning: assignment discards 'const' qualifier from pointer target type
       if (e == b) prepend = "a";
                           ^
mrbgems/mruby-string-ext/src/string.c:308:27: warning: assignment discards 'const' qualifier from pointer target type
       if (e == b) prepend = "A";
                           ^
mrbgems/mruby-bin-mruby/tools/mruby/mruby.c: In function 'main':
mrbgems/mruby-bin-mruby/tools/mruby/mruby.c:213:13: warning: assignment discards 'const' qualifier from pointer target type
     cmdline = args.cmdline ? args.cmdline : "-";
             ^
mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c: In function 'print_breakpoint':
mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c:159:3: warning: initialization discards 'const' qualifier from pointer target type
   char* enable_letter[] = {BREAK_INFO_MSG_DISABLE, BREAK_INFO_MSG_ENABLE};
   ^
mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c:159:3: warning: initialization discards 'const' qualifier from pointer target type