Makefile: Added meaningful error when luajit submodule is missing.

Makefile

@@ -10,14 +10,19 @@ LUAJIT_CFLAGS := -DLUAJIT_USE_PERFTOOLS -DLUAJIT_USE_GDBJIT
 all: $(LUAJIT_O)
 	cd src && $(MAKE)
 
-$(LUAJIT_O): deps/luajit/Makefile
+$(LUAJIT_O): check_luajit deps/luajit/Makefile
 	echo 'Building LuaJIT\n'
 	(cd deps/luajit && \
 	 $(MAKE) PREFIX=`pwd`/usr/local \
 	         CFLAGS="$(LUAJIT_CFLAGS)" && \
 	 $(MAKE) DESTDIR=`pwd` install)
 	(cd deps/luajit/usr/local/bin; ln -fs luajit-2.1.0-alpha luajit)
 
+check_luajit:
+	@if [ ! -f deps/luajit/Makefile ]; then \
+	    echo "Can't find deps/luajit/. You might need to: git submodule update --init"; exit 1; \
+	fi
+
 clean:
 	(cd deps/luajit && $(MAKE) clean)
 	(cd src; $(MAKE) clean)