Fix OSX build failure from latest XCode update.

Fix OSX build failure from latest XCode update.

To begin with, the toolchain no longer lets me build for x86-32 -
apparently MacOS is now 64-bit only.

Also, the linker now gives an error about a missing libgcc_s variant
for -mmacosx-version-min=10.4, and indeed 10.5. So I've bumped the
minimum supported OS version to 10.6.

That in turn required some changes in osx.m itself, because bumping
the min OS version caused some API deprecations to show up. Luckily I
turned out to have left myself a comment some time ago telling me what
I was going to need to do about one of them :-)