bug#21753: git tip bootstrap failure: custom needs pcase

[Sam Steingold]

the current git tip, "make bootstrap" fails with:

./temacs --batch --load loadup bootstrap
Loading loadup.el (source)...
Using load-path (/home/sds/src/emacs/trunk/lisp
/home/sds/src/emacs/trunk/lisp/emacs-lisp
/home/sds/src/emacs/trunk/lisp/language
/home/sds/src/emacs/trunk/lisp/international
/home/sds/src/emacs/trunk/lisp/textmodes
/home/sds/src/emacs/trunk/lisp/vc)
Loading emacs-lisp/byte-run (source)...
Loading emacs-lisp/backquote (source)...
Loading subr (source)...
Loading version (source)...
Loading widget (source)...
Loading custom (source)...
Symbol’s function definition is void: pcase

indeed, custom.el:custom-declare-variable uses pcase which is loaded
by loadup only later.

-- 
Sam Steingold <http://sds.podval.org> <http://www.childpsy.net/>