libsdl2 can build on 10.6, but instructions a bit vague

[kenneth.f.cunningham@…]

This one might be nitpicky I suppose, but it stuck me for while.

libsdl2 does build and run nicely on 10.6, but indeed, as the instructions say "Building $name requires OS X 10.7 SDK or later"

At first, I thought this was a no-go on 10.6, and it wasn't initially obvious to me that what was needed was to copy the MacOSX10.7.sdk from another machine to ${developer_dir}/SDKs/MacOSX10.7.sdk, and that only this exact SDK would work. Then the build on 10.6 goes fine.

How about something like:

error "Building $name on MacOSX 10.6 requires the MacOSX10.7.sdk to be present in ${developer_dir}/SDKs/"