make binary: allow custom config flags

For example, to cross-compile from my OS X laptop for Raspberry Pi, you would
do something like:

  $ make binary BINARYNAME=node-v`python tools/getnodeversion.py`-linux-arm-pi \
      DESTCPU=arm CONFIG_FLAGS="--dest-os=linux"

Makefile

@@ -278,7 +278,7 @@ tar: $(TARBALL)
 $(BINARYTAR): release-only
 	rm -rf $(BINARYNAME)
 	rm -rf out/deps out/Release
-	./configure --prefix=/ --without-snapshot --dest-cpu=$(DESTCPU)
+	./configure --prefix=/ --without-snapshot --dest-cpu=$(DESTCPU) $(CONFIG_FLAGS)
 	$(MAKE) install DESTDIR=$(BINARYNAME) V=$(V) PORTABLE=1
 	cp README.md $(BINARYNAME)
 	cp LICENSE $(BINARYNAME)