message: Add detection of current OS
  Set current OS as default target OS.
diff: @@ -9,6 +9,10 @@ NAS2DINCLUDEDIR := $(NAS2DDIR)
  NAS2DLIBDIR := $(NAS2DDIR)lib/
  NAS2DLIB := $(NAS2DLIBDIR)libnas2d.a
  +# Determine OS (Linux, Darwin, ...)
  +CURRENT_OS := $(shell uname 2>/dev/null || echo Unknown)
  +TARGET_OS ?= $(CURRENT_OS)
  +
  CPPFLAGS := $(CPPFLAGS_EXTRA)
  CXXFLAGS_WARN := -Wall -Wextra -Wpedantic -Wno-unknown-pragmas -Wnull-dereference -Wold-style-cast -Wcast-qual -Wcast-align -Wdouble-promotion -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)
  