Enable -Wcast-qual when compiling the valgrind source.
Testcases are not compiled with -Wcast-qual.
Introduce CONST_CAST macro to work around in the few spots
where a cast that drops type qualifiers is needed.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14652 a5019735-40e9-0310-863c-91ae7b9d1cf9