disable tautological compare in clang

disable the tautological compare warning in clang as crypto++ triggers this
warning and they are ignoring it
disable tautolgical compare warning in clang

disable the tautological compare warning as it is triggered by crypto++ using
unsigned ints in comparisons to zero in template code. crypto++ do not show
any indication of fixing it.

this commit does add a dependency on grep but that should not be a major
problem