We have raised the compiler requirement from time to time.
With commit cafa0010

 ("Raise the minimum required gcc version
to 4.6"), the minimum for GCC is 4.6 now.

The -Wno-pointer-sign has been here since the pre-git era. It is
documented in the GCC 4.6 manual, and it is recognized by Clang
and ICC as well.

Let's rip off the cc-disable-warning switch, and see if somebody
complains about it.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>