Bootstrap failure

[Romain Francoise]

Sometime in the last three weeks a regression was introduced which
leads a to build failure when bootstrapping, as follows:

| Loading files...
| Loading cus-face...
| Loading faces...
| Loading minibuffer...
| Loading button...
| Loading startup...
| Invalid function: "/home/romain/src/emacs/emacs.git/lisp/startup.elc"
| make[1]: *** [bootstrap-emacs] Error 255
| make[1]: Leaving directory `/home/romain/src/emacs/emacs.git/src'
| make: *** [src] Error 2

I can reliably reproduce it with the following steps:

 1. Start from a clean checkout
 2. ./configure && make
 3. make distclean
 4. ./configure && make

Step four fails with the message above. When in that situation,
doing 'make src' fails, but 'make -C src' works. This probably
points to a problem with the handling of BOOTSTRAPEMACS, but I'm not
sure what.