Patch to prevent requiring memchr at compile time

See http://d.puremagic.com/issues/show_bug.cgi?id=10078 for full description.

If the std.string.indexOf(Char[], dchar, CaseSensitive) method is called at compile time with an ASCII dchar, it will fail:

    /usr/share/dmd/src/phobos/std/string.d(345): 
        Error: memchr cannot be interpreted at compile time, 
        because it has no available source code