build: don't auto-destroy existing configuration

Don't run configure when the configure script has been touched. Doing so
would be okay if the Makefile passed the original arguments to configure
but it doesn't - it runs configure without any arguments, effectively
destroying the current configuration.

Remove this misfeature and instead print an error message telling the
user to (re-)run configure.

Makefile

@@ -53,7 +53,11 @@ else
 endif
 
 config.gypi: configure
-	$(PYTHON) ./configure
+	if [ -f $@ ]; then
+		$(error Stale $@, please re-run ./configure)
+	else
+		$(error No $@, please run ./configure first)
+	fi
 
 install: all
 	$(PYTHON) tools/install.py $@ '$(DESTDIR)' '$(PREFIX)'