Make ENV.O4 a no-op

On older Apple compilers "-O4" is known to cause build errors. On recent
clang, it's the same as "-O3" and you have to pass "-O3 -flto" to get
the old behavior.
ENV.{cc,cxx} setters already set OBJC and OBJCXX
We don't use -Qunused-arguments anymore
pebble-sdk: remove obsoleted setup.py/clang workaround