makefile-dependency-regex

makefile-dependency-regex is a variable defined in `make-mode.el'.
Its value is

"^\\(\\(?:\\$\\(?:[({]\\(?:\\$\\(?:[({]\\(?:\\$\\(?:[^({]\\|.[^\n$#})]+?[})]\\)\\|[^\n$#)}]\\)+?[})]\\|[^({]\\)\\|[^\n$#)}]\\)+?[})]\\|[^({]\\)\\|[^\n$#:=]\\)+?\\)\\(:\\)\\(?:[ 	]*$\\|[^=\n]\\(?:[^#\n]*?;[ 	]*\\(.+\\)\\)?\\)"

Documentation:
Regex used to find dependency lines in a makefile.