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

[David DeHaven]

>> * Why remove MACOSX_VERSION_MIN=@MACOSX_VERSION_MIN@? I believe we still use this in some closed makefiles. Or is the idea that we instead will force the sdk name to 10.7? If so, then we need to still leave this in until every user (RE) has switched properly.
>> 
>> I moved all that to closed, it's in the internal review for those changes. The value is now set in custom-spec.gmk instead of spec.gmk. It was not used anywhere in OpenJDK that I could find. When deploy/install get revamped we can just remove it entirely, that'll be easier if it's in closed.
>> 
>> It's referenced in make/bsd/makefiles/gcc.make, but that entire block is skipped if spec.gmk exists (confirmed by throwing garbage in there), and it defaults to 10.7.0 anyways.
> 
> Many/most/some (?) Hotspot devs still typically build without configure so there is no spec.gmk for them.

So they weren't even using it, since MACOSX_VERSION_MIN was defined in spec.gmk.

-DrD-