Allow not deleting the config.cache file with "make FAST=true bootstrap"

* Makefile.in: Add some commentary about make bootstrap,
./configure -C and FASTo
(top_bootclean): Don't delete config.cache here.
(top_distclean): Delete it here instead.
(bootstrap-clean): Allow not deleting the cache file.

* admin/emake (cores): Adjust.

Makefile.in

@@ -51,7 +51,15 @@
 #
 # make bootstrap
 #      Removes all the compiled files to force a new bootstrap from a
-#      clean slate, and then build in the normal way.
+#      clean slate, and then build in the normal way.  If the FAST Make
+#      variable is set, then the config.cache file isn't removed.  This
+#      allows you to say
+#
+#      ./configure -C
+#      make FAST=true bootstrap
+#
+#      and use the cached results from the configure run, which is much
+#      faster.
 #
 # make docs
 #      Make Emacs documentation files from their sources; requires makeinfo.
@@ -936,7 +944,7 @@ clean: $(clean_dirs:=_clean) clean-gsettings-schemas
 ### 'bootclean'
 ###      Delete all files that need to be remade for a clean bootstrap.
 top_bootclean=\
-	rm -f config.cache config.log ${srcdir}/doc/man/emacs.1
+	rm -f config.log ${srcdir}/doc/man/emacs.1
 
 ### 'distclean'
 ###      Delete all files from the current directory that are created by
@@ -946,7 +954,7 @@ top_bootclean=\
 ###      distribution.
 top_distclean=\
 	${top_bootclean}; \
-	rm -f config.status config.log~ \
+	rm -f config.cache config.status config.log~ \
 	  Makefile makefile lib/gnulib.mk ${SUBDIR_MAKEFILES}
 
 distclean_dirs = $(clean_dirs) leim lisp admin/grammars
@@ -966,6 +974,9 @@ bootstrap-clean: $(distclean_dirs:=_bootstrap-clean)
 	rm -rf ${srcdir}/info
 	rm -f ${srcdir}/etc/refcards/emacsver.tex
 	rm -rf native-lisp/ lisp/leim/ja-dic/
+ifndef FAST
+	rm -f config.cache
+endif
 	${top_bootclean}
 
 ### 'maintainer-clean'

admin/emake

@@ -20,7 +20,7 @@ if [ -f /proc/cpuinfo ]; then
 	sed 's/^[0-9]*/+/')))
 fi
 
-make -j$cores "$@" 2>&1 | \
+make FAST=true -j$cores "$@" 2>&1 | \
 sed -u 's# \.\./\.\./# #
 s# \.\./# #
 s#^Configuring local git # Configuring local git #