use install in makefile instead of cp

depending on umask, cp won't install the binaries with execute bit set. using
install lets us explicitly state that they should have 755 permissions.

Makefile

@@ -13,8 +13,8 @@ all: clean build
 build: amqpspawn amqpsend
 
 install: bin/amqpspawn bin/amqpsend
-	cp -f bin/amqpspawn $(AMQPTOOLS_INSTALLROOT)/
-	cp -f bin/amqpsend $(AMQPTOOLS_INSTALLROOT)/
+	install -D -m0755 bin/amqpspawn $(AMQPTOOLS_INSTALLROOT)/amqpspawn
+	install -D -m0755 bin/amqpsend $(AMQPTOOLS_INSTALLROOT)/amqpsend
 
 uninstall:
 	rm -f $(AMQPTOOLS_INSTALLROOT)/amqpspawn