all_hosts = Skeleton Linux FreeBSD

ifeq ($(OS),)
SYS := $(shell gcc -dumpmachine)
ifeq ($(SYS),x86_64-linux-gnu)
OS := Linux
else ifeq ($(findstring freebsd,$(SYS)),freebsd)
OS := FreeBSD
else
$(error Unsupported platform: $(SYS))
endif
else
ifeq ($(findstring $(OS),$(all_hosts)),)
$(error Unsupported platform: $(OS))
endif
endif

OS_MACRO := $(shell echo $(OS) | tr '[:lower:]' '[:upper:]' | tr '-' '_')