The define for g_utf8_next_char(p) includes a not needed final cast to
(char *). In fact, this cast has the adverse effect of causing a warning
if p is a (const char *) with gcc/clang compiler option -Wcast-qual.
So lets remove the not needed cast and add option -Werror=cast-qual
to glib/tests/utf8-pointer.c which uses g_utf8_next_char().
Now utf8-pointer.c compiles also with compiler option -Werror=cast-qual
and passes all tests.