Fix MacOSX cmake missing symbol _clock_gettime On El Capitan: [exec] [100%] Linking C executable openssl [exec] Undefined symbols for architecture x86_64: [exec] "_clock_gettime", referenced from: [exec] _app_timer_real in apps_posix.c.o [exec] ld: symbol(s) not found for architecture x86_64 [exec] clang: error: linker command failed with exit code 1 (use -v to see invocation) [exec] make[2]: *** [apps/openssl/openssl] Error 1 [exec] make[1]: *** [apps/openssl/CMakeFiles/openssl.dir/all] Error 2 [exec] make: *** [all] Error 2