Makefile.in strikes again:
# We run make-docfile twice because the command line may get too long # on some systems. The sed command operating on lisp.mk also reduces # the length of the command line. Unfortunately, no-one has any idea # exactly how long the maximum safe command line length is on all the # various systems that Emacs supports. Obviously, the length depends # on what your value of $srcdir is. If the length restriction goes # away, lisp.mk can be merged back into this file.