bug#24974: CANNOT_DUMP build assumes Emacs is already installed

[Paul Eggert]

The CANNOT_DUMP build procedure is confused: it assumes that the current version 
of Emacs is already installed, and Emacs builds can fail (or be subtly wrong) 
when this assumption is not true. To reproduce the problem, pick a directory 
that doesn't exist ("/tmp/prefix" in the example below) and configure and build 
this way:

./configure --prefix=/tmp/prefix CANNOT_DUMP=yes
make bootstrap

On my platform (Ubuntu 16.04 x86-64) the build fails as follows:

ln -f temacs bootstrap-emacs
make -C ../lisp compile-first EMACS="../src/bootstrap-emacs"
make[3]: Entering directory '/home/eggert/src/gnu/emacs/static-checking/lisp'
  ELC      emacs-lisp/macroexp.elc
Warning: Lisp directory '/tmp/prefix/share/emacs/26.0.50/lisp': No such file or 
directory
Cannot open load file: No such file or directory, loadup.el
Makefile:282: recipe for target 'emacs-lisp/macroexp.elc' failed

The full command that fails (abbreviated "ELC emacs-lisp/macrorexp.elc above) 
is:

EMACSLOADPATH= '../src/bootstrap-emacs' -batch --no-site-file --no-site-lisp -l 
autoload \
   --eval "(setq generate-autoload-cookie \";;;###cal-autoload\")" \
   --eval "(setq generated-autoload-file (expand-file-name (unmsys--file-name 
\"calendar/cal-loaddefs.el\")))" \
   -f batch-update-autoloads ./calendar

Running strace on this command reveals that it attempts to open only:

/tmp/prefix/share/emacs/26.0.50/lisp/loadup.el.elc
/tmp/prefix/share/emacs/26.0.50/lisp/loadup.el.el
/tmp/prefix/share/emacs/26.0.50/lisp/loadup.el

and it never attempts to open loadup.el in the current directory, which is 
what's needed here.

By the way, why does Emacs try to open ".../loadup.el.elc"? Isn't that a waste 
of time?