move libc related stuff out of platform.h

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/login.c b/libbb/login.c
index 98e641c..ba9f4d2 100644
--- a/libbb/login.c
+++ b/libbb/login.c
@@ -62,7 +62,8 @@
 			case 'm':
 				outbuf = uts.machine;
 				break;
-#ifndef HAVE_NO_UTSNAME_DOMAINNAME
+/* The field domainname of struct utsname is Linux specific. */
+#if defined(__linux__)
 			case 'D':
 			case 'o':
 				outbuf = uts.domainname;