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The core issue here is the weak linking of the symbol, and in certain environments--for instance, using the latest Xcode (8.1) with the latest SDK (10.12)--os_unfair_lock may resolve even though you're compiling on a host that doesn't support it (10.11). We can use the availability macros to circumvent this problem, and detect that we're not compiling for a target that is going to support them and error out at compile time. The other alternative is to do a runtime check, but that presents issues for cross-compiling.
jemalloc is a general purpose malloc(3) implementation that emphasizes fragmentation avoidance and scalable concurrency support. jemalloc first came into use as the FreeBSD libc allocator in 2005, and since then it has found its way into numerous applications that rely on its predictable behavior. In 2010 jemalloc development efforts broadened to include developer support features such as heap profiling and extensive monitoring/tuning hooks. Modern jemalloc releases continue to be integrated back into FreeBSD, and therefore versatility remains critical. Ongoing development efforts trend toward making jemalloc among the best allocators for a broad range of demanding applications, and eliminating/mitigating weaknesses that have practical repercussions for real world applications. The COPYING file contains copyright and licensing information. The INSTALL file contains information on how to configure, build, and install jemalloc. The ChangeLog file contains a brief summary of changes for each release. URL: http://jemalloc.net/