Generating compile_commands.json for Traffic Server on macOS

[Masaori Koshiba]

For people who looking for how to generate compile_commands.json for clang
based tools like rtags on macOS.

https://cwiki.apache.org/confluence/display/TS/macOS#macOS-GeneratesacompilationdatabaseforclangtoolingwithBear

Enjoy!
Masaori