message: Add Flags.EXTRA variables to makefile
  This allows an easy way to specify custom compiler flags from the
  command line, while building through `make`. This can be used to add
  instrumentation flags for things like code coverage, or to change the
  optimization level.
diff: @@ -9,9 +9,9 @@ NAS2DINCLUDEDIR := $(NAS2DDIR)include/
  NAS2DLIBDIR := $(NAS2DDIR)lib/
  NAS2DLIB := $(NAS2DLIBDIR)libnas2d.a
  -CXXFLAGS := -std=c++17 -g -Wall -Wno-unknown-pragmas -I$(NAS2DINCLUDEDIR) $(shell sdl2-config --cflags)
  -LDFLAGS := -L$(NAS2DLIBDIR) $(shell sdl2-config --libs)
  -LDLIBS := -lnas2d -lSDL2 -lSDL2_image -lSDL2_mixer -lSDL2_ttf -lphysfs -lGL -lGLEW
  +CXXFLAGS := $(CXXFLAGS.EXTRA) -std=c++17 -g -Wall -Wno-unknown-pragmas -I$(NAS2DINCLUDEDIR) $(shell sdl2-config --cflags)
  +LDFLAGS := $(LDFLAGS.EXTRA) -L$(NAS2DLIBDIR) $(shell sdl2-config --libs)
  +LDLIBS := $(LDLIBS.EXTRA) -lnas2d -lSDL2 -lSDL2_image -lSDL2_mixer -lSDL2_ttf -lphysfs -lGL -lGLEW
  DEPFLAGS = -MT $@ -MMD -MP -MF $(OBJDIR)$*.Td
  