Tweak implementation of -Wwrite-strings to better match the behavior of current GCCs:

 * In C, as before, if the "warning flag" is enabled, warnings are produced by
   forcing string literals to have const-qualified types (the produced warnings
   are *not* -Wwrite-strings warnings). However, more recent GCCs (at least 4.4
   onwards) now take -w into account here, so we now do the same.

 * In C++, this flag is entirely sane: it behaves just like any other warning
   flag. Stop triggering -fconst-strings here. This is a bit cleaner, but there's
   no real functionality change except in the case where -Xclang -fno-const-strings
   is also specified.