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.