Make OS X's lame UNIX happy with mod_mongo Makefile

Apparently OS X ships a sed that requires an argument to -i.

src/mod/applications/mod_mongo/Makefile

@@ -26,7 +26,7 @@ $(MONGO_CXX_DRIVER_SRC):
 
 $(LIBMONGOCLIENT_A): $(MONGO_CXX_DRIVER_SRC)
 	(cd $(MONGO_CXX_DRIVER_SRC) && \
-		sed -i SConstruct -e "s|Environment()|Environment(ENV = {'PATH' : os.environ.get('PATH','/bin:/usr/bin'),'CFLAGS' : os.environ.get('CFLAGS',''),'CXXFLAGS' : os.environ.get('CXXFLAGS','')})|" && \
+		sed -i'' SConstruct -e "s|Environment()|Environment(ENV = {'PATH' : os.environ.get('PATH','/bin:/usr/bin'),'CFLAGS' : os.environ.get('CFLAGS',''),'CXXFLAGS' : os.environ.get('CXXFLAGS','')})|" && \
 		echo "env.Append(PATH=os.environ.get('PATH','/bin:/usr/bin'))" >> SConstruct && \
 		scons)
 	$(TOUCH_TARGET)