Don't use -Wpointer-arith if it produces excessive warnings

[Dmitry Timoshkov]

"Mike McCormack" <mike at codeweavers.com> wrote:

> On Slackware 8.1, this caused a screenful of warnings for *every* file 
> compiled, which makes it difficult to see important warnings.

Same on RH 6.2. But it's actually bug(s) in string.h and a better way IMHO
is just fix it, instead of silencing warnings if a user is *really* annoyed
by that avalanche of warnings.

-- 
Dmitry.