Makefile: Update a comment

Makefile

@@ -47,8 +47,10 @@ CONFIG_FILES = build/make/Makefile src/bin/sage-env-config build/bin/sage-build-
 # SPKG_COLLECT_FILES contains all files that influence the SAGE_SPKG_COLLECT macro
 SPKG_COLLECT_FILES = build/pkgs/*/type build/pkgs/*/package-version.txt build/pkgs/*/dependencies build/pkgs/*/requirements.txt build/pkgs/*/checksums.ini build/pkgs/*/spkg-install
 
-# If configure was run before, rerun it with the old arguments.
-# Otherwise, run configure with argument $PREREQ_OPTIONS.
+# If configure was not run before, complain.
+# If configure is newer than the files it generated (we test build/make/Makefile),
+#    we regenerate config.status by running the "config.status --recheck".
+# Either way we regenerate the generated files by calling "config.status".
 build/make/Makefile: configure $(SPKG_COLLECT_FILES) $(CONFIG_FILES:%=%.in)
 	@if [ -x config.status ]; then		\
 	    case '$?' in			\