CMake Quick Reference

Include out-of-tree source directory

add_subdirectory(path/to/my/ext/src path/to/binary)

Not providing binary_dir when calling add_subdirectory() results in the following error message:

  add_subdirectory not given a binary directory but the given source
  directory "path/to/my/ext/src" is not a subdirectory of
  "path/to/my/project/root"

ref: https://stackoverflow.com/questions/35260552/how-do-i-explicitly-specify-an-out-of-tree-source-in-cmake