Improved -Wdeprecated-declarations warning suppression

Don't depend on OS X SDK version anymore, since my 10.6 dashboard machine also has this warning;
instead always suppress the warning on OS X.  If gcc is new enough, suppress only within
the header, otherwise we're forced to suppress for the remainer of the translation unit.

Change-Id: Ibfa92d5895cb59c20dbc2c894e56c9e875e6fc9a
Merge topic 'improved-deprecation-suppression' into master

d094173f Improved -Wdeprecated-declarations warning suppression