clock_gettime(3) is found in libc (/usr/lib/system/libsystem_c.dylib) on macOS 10.12+. There's no seperate librt. AC_SEARCH_LIBS(clock_gettime, rt, ...) does not guarantee librt is present. It only attempts to link against librt if clock_gettime isn't available when linking aginast no extra libraries; but since it's already in libc, the -lrt step is simply skipped.
Fix "no member named 'st_atim' in 'struct stat'" on macOS
Cool, thanks for the quick reaction!
but I got the impression that you already released the homebrew ffmpeg 3.0 release...
I do not understand how you expect a dependency that has newer version that breaks...
Build failure against FFmpeg 3.0