message: Add `CPPFLAGS` variable
  This might be used to add an additional include folder for a different
  version of a dependency stored in a non-standard folder.
  This makes the available variables more consistent with other projects.
diff: @@ -9,13 +9,14 @@ NAS2DINCLUDEDIR := $(NAS2DDIR)include/
  NAS2DLIBDIR := $(NAS2DDIR)lib/
  NAS2DLIB := $(NAS2DLIBDIR)libnas2d.a
  +CPPFLAGS := $(CPPFLAGS.EXTRA)
  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
  -COMPILE.cpp = $(CXX) $(DEPFLAGS) $(CXXFLAGS) $(TARGET_ARCH) -c
  +COMPILE.cpp = $(CXX) $(DEPFLAGS) $(CPPFLAGS) $(CXXFLAGS) $(TARGET_ARCH) -c
  POSTCOMPILE = @mv -f $(OBJDIR)$*.Td $(OBJDIR)$*.d && touch $@
  SRCS := $(shell find $(SRCDIR) -name '*.cpp')
  