As you can see in the message, I have to see config.log to figure out the reason.

@kewlbear Thank you for your reply. But I don't know where to find the "config.log" file.

You can do the following:

find -name config.log

Below is the last part of config.log. I have emailed the config.log file to ffmpeg-user@ffmpeg.org
My Xcode is version 9.2
"xcrun -sdk iphoneos clang -arch arm64 -mios-version-min=6.0 -fembed-bitcode -c -o /var/folders/3s/r_mvg2dd7hz5nyd5my5jztrh0000gp/T/ffconf.K8OIrHVS/test.o /var/folders/3s/r_mvg2dd7hz5nyd5my5jztrh0000gp/T/ffconf.K8OIrHVS/test.c
xcrun: error: SDK "iphoneos" cannot be located
xcrun -sdk iphoneos clang -arch arm64 -mios-version-min=6.0 -fembed-bitcode -o /var/folders/3s/r_mvg2dd7hz5nyd5my5jztrh0000gp/T/ffconf.K8OIrHVS/test /var/folders/3s/r_mvg2dd7hz5nyd5my5jztrh0000gp/T/ffconf.K8OIrHVS/test.o
xcrun: error: SDK "iphoneos" cannot be located
ld: warning: ignoring file /usr/lib/libSystem.dylib, missing required architecture arm64 in file /usr/lib/libSystem.dylib (2 slices)
ld: dynamic main executables must link with libSystem.dylib for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
C compiler test failed."

After googling “xcrun: error: SDK "iphoneos" cannot be located”, I’m guessing some reasons:

1. Did you rename Xcode.app?
2. Did you install command line tools?

I fixed “xcrun: error: SDK "iphoneos" cannot be located” by referring to "gomatcha/matcha#6"
with "sudo xcode-select --switch /Applications/Xcode.app"
Thank you for your help again.

Glad I could help.