#2 OS X dylib linker fix

[Brendan Cully]

I believe compatibility_version and current_version are swapped in your
OS X dylib linker rule. I've been using this patch for some time in the fink
port.

[Brendan Cully]

Swap compatibility_version with current_version

[Mikio Hirabayashi]

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Thanks. I'll apply it in the next version.