Fix wonky Darwin linker test in configure.ac
Fix wonky Darwin linker test in configure.ac

On Ubuntu 12.04, this check for the Darwin linker was passing, when it should not have. This resulted in a link failure when the build system tried to pass the Darwin-specific (?) `-compatibility_version` flag to the linker.

My autotools knowledge is practically non-existent, but this patch seems to make the test pass on OS X and fail on Linux, which I'm assuming is what's supposed to happen.

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