[CMake] Building fail on Mac M1.

[czkkkkkk]

🐛 Bug

When building on Mac M1, the CC compiler cannot find "pthread.h".

This is bacause #5497 uses CMAKE_C_COMPILER for building libxsmm, which is "/Library/Developer/CommandLineTools/usr/bin/cc" on Mac M1.

However, if I simply remove this change and use "/usr/bin/cc", other errors popup.

To Reproduce

Expected behavior

Environment

• DGL Version (e.g., 1.0): master
• Backend Library & Version (e.g., PyTorch 0.4.1, MXNet/Gluon 1.3):
• OS (e.g., Linux): mac m1
• How you installed DGL (conda, pip, source):
• Build command you used (if compiling from source):
• Python version:
• CUDA/cuDNN version (if applicable):
• GPU models and configuration (e.g. V100):
• Any other relevant information:

Additional context

[itaraban]

@czkkkkkk could you please check my changes(#5626) on your machine?

[czkkkkkk]

@czkkkkkk could you please check my changes(#5626) on your machine?

Sure! Thanks for the fix.