Change how backup/restore make task works.

Now we write $SRC/.config.mk.last to store the most recent configure
variables for the backup and restore task. Also, they no longer fail if
they can't find the variables, and will skip the backup/restore stages
and keep going.

.gitignore

@@ -13,6 +13,7 @@
 .DS_Store
 servo-test
 Servo.app
+.config.mk.last
 build*
 objdir
 parser.out

Makefile.backup

@@ -7,11 +7,12 @@
 #
 # restore-rust is run after configure and before invoking make. if it fails, we must abort.
 
-include config.mk
+-include .config.mk.last
 
-.PHONY: backup-rust restore-rust
+.PHONY: backup-rust
 backup-rust:
-	-mv $(CFG_BUILD_DIR)src/compiler/rust ../$(CFG_TARGET)
+	@if [ ! -z "$(CFG_BUILD_DIR)" ]; then echo "Backing up Rust"; mv $(CFG_BUILD_DIR)src/compiler/rust ../$(CFG_TARGET); else echo "Skipping Rust backup"; fi
 
+.PHONY: restore-rust
 restore-rust:
-	if [ -d ../$(CFG_TARGET) ]; then rm -rf $(CFG_BUILD_DIR)src/compiler/rust; mv ../$(CFG_TARGET) $(CFG_BUILD_DIR)src/compiler/rust; fi
+	@if [ ! -z "$(CFG_BUILD_DIR)" -a -d ../$(CFG_TARGET) ]; then echo "Restoring Rust from backup"; rm -rf $(CFG_BUILD_DIR)src/compiler/rust; mv ../$(CFG_TARGET) $(CFG_BUILD_DIR)src/compiler/rust; else echo "Skipping Rust restore"; fi

configure

@@ -565,6 +565,7 @@ putvar CFG_ENABLE_DEBUG_SKIA
 msg
 copy_if_changed ${CFG_SRC_DIR}Makefile.in ${CFG_BUILD_HOME}Makefile
 move_if_changed ${CFG_BUILD_HOME}config.tmp ${CFG_BUILD_HOME}config.mk
+copy_if_changed ${CFG_BUILD_HOME}config.mk ${CFG_SRC_DIR}/.config.mk.last
 rm -f ${CFG_BUILD_HOME}config.tmp
 touch ${CFG_BUILD_HOME}config.stamp