build system: remove -wcast-qual

Generating warnings when casting const away leads to tight constraints
on the code if one wants to avoid warnings. This is especially true for
generic code that is supposed to work with both const and non const.
These tight constrains cause people to waste time trying to find a
way to write code so it doesnt generate any warning, while people
should rather spend their time thinking on how to write fast,
clean, maintainable and bug free code.
Removing this class of warnings fixes this issue.
Approved-by: Nicolas George <nicolas.george@normalesup.org>
Approved-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -3333,7 +3333,6 @@ check_cflags -Wdisabled-optimization
 check_cflags -Wpointer-arith
 check_cflags -Wredundant-decls
 check_cflags -Wno-pointer-sign
-check_cflags -Wcast-qual
 check_cflags -Wwrite-strings
 check_cflags -Wtype-limits
 check_cflags -Wundef