RFR: [9] 8043340 & 8043591: [macosx] Build system issues

[David DeHaven]

> Looks pretty good. A couple of questions still:
> 
> * Where is the AC_SUBST for SDKROOT?
> 
> * Could we get the "Checking for Xcode sdk name" output to only print on macosx?

Would this be OK instead?

    AC_MSG_CHECKING([for sdk name])
    AC_MSG_RESULT([$SDKNAME])

Otherwise it might make sense to just move the whole arg into the macosx block too.

-DrD-