[Build] Added MAKE_FLAGS variable to Makefile

+ Mainly too provide -j parallel execution option

Makefile

@@ -5,20 +5,21 @@ CMAKE_COMMAND := cmake
 else
 CMAKE_COMMAND := ${CMAKE_COMMAND}
 endif
+MAKE_FLAGS :=
 
 .PHONY: all checkout compile
 
 all: checkout
 	mkdir -p build && cd build && $(CMAKE_COMMAND) ${CMAKE_FLAGS} ..
-	cd build && $(MAKE)
+	cd build && $(MAKE) ${MAKE_FLAGS}
 
-checkout: 
+checkout:
 	git submodule init
 	git submodule update --init --recursive
 
 compile:
 	mkdir -p build && cd build && $(CMAKE_COMMAND) ${CMAKE_FLAGS} ..
-	cd build && $(MAKE)
+	cd build && $(MAKE) ${MAKE_FLAGS}
 
 clean:
 	rm -rf build