+# check the OS is supported, rather than going any further
+supported_exec_envs = ['freebsd', 'linux', 'windows']
+exec_env = host_machine.system()
+if not supported_exec_envs.contains(exec_env)
+ error('unsupported system type "@0@"'.format(exec_env))
+endif
+
+# define a handy variable for checking which OS we have.
+# gives us "is_windows", "is_freebsd" and "is_linux"
+foreach env:supported_exec_envs
+ set_variable('is_' + env, exec_env == env)
+endforeach
+