Automating the generation of compile_commands.json about emacs-ccls

[MaskRay]

compile_commands.json should not be generated automatically.

• Where to put compile_commands.json: I usually prefer $project/Debug and for CMake-based projects I use cmake -H. -BDebug -DCMAKE_EXPORT_COMPILE_COMMANDS=YES <other options>. This operation creates some files that may not be wanted.
• The various -D* options. They cannot be known beforehand.
• compile_commands.json is not the only available choice: after the merge of MaskRay/ccls#171 , .ccls can augment compile_commands.json
• It is not straightforward to check the existence of compile_commands.json. I don't want to locate the project root with compile_commands.json (lots of other plugins do things like this) because:

proj
 Debug
  compile_commands.json
  generate.h        # this belongs to proj, not proj/Debug 
 compile_commands.json -> Debug/compile_commands.json