dist: add clang specific -Wno options

dist: add clang specific -Wno options

Clang 3.1 has more warnings enabled by default than GCC.
Combined with -Werror, they cause the build to fail
unnecessarily.  if the name of our compiler ends in "clang",
add the necessary extra -Wno options.  Ideally we would add
these flags unconditionally, as GCC is supposed to ignore
unknown -Wno flags, but apple's llvm-gcc doesn't.

Fixes #2878.

R=rsc, dave
CC=golang-dev
https://golang.org/cl/5673055