* configure.ac (with-build-config): Document.  Handle without.
Handle missing argument.
* configure: Rebuilt.
config/ChangeLog:
* bootstrap-debug-big.mk (STAGE2_CFLAGS): Drop -gtoggle.
* bootstrap-debug-lean.mk: Update comments.
(STAGE2_CFLAGS): Likewise.
(do-compare): Don't override.
ChangeLog:
* configure.ac (with-build-config): Document.  Handle without.
Handle missing argument.
* configure: Rebuilt.