Finding the CMake build/ directory in Emacs Lisp (elisp) for compilation

find-cmake-builddir.el

; Add this function to your .emacs file and add it to the c-mode-common hook:
;   (add-hook 'c-mode-common-hook 'find-cmake-builddir)
; Afterwards the `compile-command' variable will automatically contain a command for
; building your entire CMake project by just doing `M-x compile' whenever you
; open a C/C++ file inside a CMake-managed project. Sole requirement:
; You first have to manually create a directory "build/" next to your
; CMakeLists.txt file and run CMake with the appropriate options yourself.
(defun find-cmake-builddir ()
  "Finds the build directory of a CMake project"
  (let ((cmakefile nil) (dirname default-directory))
	; Step 1: Find the CMakeFile in the parent directories
	; Abort if found or if root dir is reached.
    (while (and (not (string= dirname "/")) (not cmakefile))
      (setq cmakefile (concat (file-name-as-directory dirname) "CMakeLists.txt"))
      (if (not (file-exists-p cmakefile))
	  (progn
	    (setq cmakefile nil)
	    (setq dirname (expand-file-name (concat (file-name-as-directory dirname) ".."))))))
	; Step 2: Check if the build directory is there, and if so,
	; whether there's a Makefile in it (i.e. cmake has been run)
    (if cmakefile
	(let ((builddir (concat (file-name-directory cmakefile) "build")))
	  (if (file-exists-p builddir)
	      (let ((makefile (concat (file-name-as-directory builddir) "Makefile")))
		(if (file-exists-p makefile)
		    (set (make-local-variable 'compile-command) (concat "make -C " (shell-quote-argument builddir))))))))))