Simplify makefile

Makefile

@@ -1,3 +1,5 @@
+FILES=chronicle.cpp screen.cpp RtAudio.cpp
+
 MSYS2WIN32LIBS=-lole32 -ldsound -luser32 -lwinmm -luuid -lksuser
 
 MSYS2WINBOOSTLIBS=-lboost_filesystem-mt -lboost_system-mt
@@ -7,21 +9,21 @@ MSYS2LIBS=`pkg-config --cflags --libs sndfile rtaudio`
 
 msys232:
 	mkdir -p build/32
-	g++ chronicle.cpp RtAudio.cpp -std=c++11 $(MSYS2WIN32LIBS) $(MSYS2LIBS) $(MSYS2WINBOOSTLIBS) -lncurses -o build/32/chronicle.exe
+	g++ $(FILES) -std=c++11 $(MSYS2WIN32LIBS) $(MSYS2LIBS) $(MSYS2WINBOOSTLIBS) -lncurses -o build/32/chronicle.exe
 	cp libs/32/* build/32
 	cp README.md build/32
 	cp LICENCE build/32
 
 msys264:
 	mkdir -p build/64
-	g++ chronicle.cpp screen.cpp RtAudio.cpp -std=c++11 $(MSYS2WIN32LIBS) $(MSYS2LIBS) $(MSYS2WINBOOSTLIBS) -lncurses -o build/64/chronicle.exe
+	g++ $(FILES) -std=c++11 $(MSYS2WIN32LIBS) $(MSYS2LIBS) $(MSYS2WINBOOSTLIBS) -lncurses -o build/64/chronicle.exe
 	cp libs/64/* build/64/
 	cp README.md build/64
 	cp LICENCE build/64
 
 linux:
 	mkdir -p build/linux
-	g++ chronicle.cpp RtAudio.cpp -std=c++11 -D__LINUX_ALSA__ -lpthread $(MSYS2LIBS) $(MSYS2LINBOOSTLIBS) -lncurses -o build/linux/chronicle
+	g++ $(FILES) -std=c++11 -D__LINUX_ALSA__ -lpthread $(MSYS2LIBS) $(MSYS2LINBOOSTLIBS) -lncurses -o build/linux/chronicle
 	cp README.md build/linux
 	cp LICENCE build/linux