Merge pull request #351 from jeremy/restore-o3-cflags

# Work around warnings building Ruby 2.0 on Clang 2.x:

# pass -Wno-error=shorten-64-to-32 if the compiler accepts it.

#

# When we set CFLAGS, Ruby won't apply its default flags, though. Since clang

# builds 1.9.x and 2.x only, where -O3 is default, we can safely set that flag.

# Ensure it's the first flag since later flags take precedence.

if "${CC:-cc}" -x c /dev/null -E -Wno-error=shorten-64-to-32 &>/dev/null; then

  RUBY_CFLAGS="$RUBY_CFLAGS -Wno-error=shorten-64-to-32"

  RUBY_CFLAGS="-O3 -Wno-error=shorten-64-to-32 $RUBY_CFLAGS"

fi

if [ -z "$MAKE" ]; then