message: Add MacOS support when linking to OpenGL libraries
  Copied from NAS2D project. Also includes support for Mingw cross
  compiles for Windows (with manually specified TARGET_OS).
diff: @@ -14,11 +14,16 @@ PACKAGEDIR := $(BUILDDIR)/package/
  CURRENT_OS := $(shell uname 2>/dev/null || echo Unknown)
  TARGET_OS ?= $(CURRENT_OS)
  +Linux_OpenGL_LIBS := -lGLEW -lGL
  +Darwin_OpenGL_LIBS := -lGLEW -framework OpenGL
  +Windows_OpenGL_LIBS := -lglew32 -lopengl32
  +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)
  LDFLAGS := $(LDFLAGS_EXTRA) -L$(NAS2DLIBDIR) $(shell sdl2-config --libs)
  -LDLIBS := $(LDLIBS_EXTRA) -lnas2d -lSDL2 -lSDL2_image -lSDL2_mixer -lSDL2_ttf -lphysfs -lGL -lGLEW
  +LDLIBS := $(LDLIBS_EXTRA) -lnas2d -lSDL2 -lSDL2_image -lSDL2_mixer -lSDL2_ttf -lphysfs $(OpenGL_LIBS)
  DEPFLAGS = -MT $@ -MMD -MP -MF $(OBJDIR)$*.Td
  