message: Rename makefile `.EXTRA` flags to `_EXTRA`
  This is a more standard naming practice. In particular, when inheriting
  environment variables from the shell, a more restrictive set of
  characters is used, which does not include ".".
diff: @@ -9,10 +9,10 @@ NAS2DINCLUDEDIR := $(NAS2DDIR)include/
  NAS2DLIBDIR := $(NAS2DDIR)lib/
  NAS2DLIB := $(NAS2DLIBDIR)libnas2d.a
  -CPPFLAGS := $(CPPFLAGS.EXTRA)
  -CXXFLAGS := $(CXXFLAGS.EXTRA) -std=c++17 -Wall -Wno-unknown-pragmas -I$(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
  +CPPFLAGS := $(CPPFLAGS_EXTRA)
  +CXXFLAGS := $(CXXFLAGS_EXTRA) -std=c++17 -Wall -Wno-unknown-pragmas -I$(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
  