message: Add "debug" and "release" flags
  Set "debug" flags as default.
diff: # Source http://make.mad-scientist.net/papers/advanced-auto-dependency-generation/
  +CONFIG := debug
  +debug_CXX_FLAGS := -Og -g
  +release_CXX_FLAGS := -O3
  +CONFIG_CXX_FLAGS := $($(CONFIG)_CXX_FLAGS)
  +
  SRCDIR := OPHD/
  BUILDDIR := .build/
  OBJDIR := $(BUILDDIR)obj/
  @@ -21,7 +26,7 @@ OpenGL_LIBS := $($(TARGET_OS)_OpenGL_LIBS)
  CPPFLAGS := $(CPPFLAGS_EXTRA)
  CXXFLAGS_WARN := -Wall -Wextra -Wpedantic -Wno-unknown-pragmas -Wnull-dereference -Wold-style-cast -Wcast-qual -Wcast-align -Wdouble-promotion -Wfloat-conversion -Wshadow -Wnon-virtual-dtor -Woverloaded-virtual -Wmissing-include-dirs -Winvalid-pch -Wmissing-format-attribute $(WARN_EXTRA)
  -CXXFLAGS := $(CXXFLAGS_EXTRA) -std=c++17 $(CXXFLAGS_WARN) -I$(NAS2DINCLUDEDIR) $(shell sdl2-config --cflags)
  +CXXFLAGS := $(CXXFLAGS_EXTRA) $(CONFIG_CXX_FLAGS) -std=c++17 $(CXXFLAGS_WARN) -I$(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 $(OpenGL_LIBS)
  