Some MacOSX build related questions

[CHENG Gao]

1) In nextstep/README, it states MacOSX supported are 10.4 and up.
I suppose this is official. (It must be)

2) Considering 1), some code that checks MAC_OS_X_VERSION > 1030 or <
1040  or even < 1020 seems not necessary anymore. 

3) For same reason, is MAC_OS_X_VERSION_MAX_ALLOWED >=
MAC_OS_X_VERSION_10_4 check necessary anymore?

4) MAC_OS_X_VERSION_10_x (3, 4, 5, 6 at present) are defined in nsterm.h
and some other files. Is it necessary? Is it enough as nsgui.h does? (as below)

,----
| #ifdef NS_IMPL_COCOA
| #include <AvailabilityMacros.h>
| #endif /* NS_IMPL_COCOA */
`----

5) In nsimage.m, it has 'onTiger' check (defined in nsterm.h). If
official support version is 10.4 (Tiger) and up, this seems not
necessary anymore.