-Dvariable=value, --define variable=value
			define global macro variable

 33 /* Predefined variables: tail of link chain */
 34 static struct substvar
 35     sv_arch   = {"ARCH",   un.machine,  1, NULL },
 36     sv_cpu    = {"CPU",    processor,   1, &sv_arch},
 37     sv_host   = {"HOST",   un.nodename, 1, &sv_cpu},
 38     sv_osname = {"OSNAME", un.sysname,  1, &sv_host},
 39     sv_osrel  = {"OSREL",  un.release,  1, &sv_osname},
 40     sv_osvers = {"OSVERS", un.version,  1, &sv_osrel},
 41     sv_dollar = {"dollar", "$",         1, &sv_osvers},
 42     sv_true   = {"true",   "1",         1, &sv_dollar},
 43     sv_false  = {"false",  "0",         1, &sv_true},
 44     sv_byte   = {"byte",   endian,      1, &sv_false},
 45     sv_host2  = {"host",   host,        1, &sv_byte},
 46     sv_xhost  = {"xhost",  host,        1, &sv_host2},
 47     sv_domain = {"domain", domain,      1, &sv_xhost},
 48     sv_hostd  = {"hostd",  hostd,       1, &sv_domain
 49 };