Mac 10.6 support for future version

[Norbert Thiebaud]

As discussed recently wrt to c++11 support, it is desirable to
eventually drop 10.6 support.

With 10.9 release and Apple policy to allow free upgrade to 10.9 from
10.6/7/8, the market share
are switched dramatically in the last 6 months...
Today 10.9 + 10.8 represent about 80% of the >= 10.6 market.
by the time the version _after_ 4.3 goes GA that number is going to be
much higher.

So I would recommend that we declare 10.6 and 10.7 'deprecated' as we
announce 4.3.. with the intent to switch our baseline to 10.8/64 bits
by the release after that... (tentatively 4.4)

Norbert

PS: there is other reasons to move the baseline beside c++1 support.
first and foremost is that we have only 1 box that is still a native
10.6... so we are one hardware failure away from not being able to
build it at all with the current resources.
otoh we do have 3 or 4 mac that can do 10.8 64 bits build.