Fix non-Linux builds

Various tools are Linuxish and should thus only attempted to build on
Linux only. Some features are also Linux-only.

Also, libresolv is used on all GNU platforms, notably GNU/Hurd and
GNU/kfreeBSD.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/util-linux/pivot_root.c b/util-linux/pivot_root.c
index ecc8911..41f29da 100644
--- a/util-linux/pivot_root.c
+++ b/util-linux/pivot_root.c
@@ -11,6 +11,7 @@
 //config:config PIVOT_ROOT
 //config:	bool "pivot_root (1.1 kb)"
 //config:	default y
+//config:	select PLATFORM_LINUX
 //config:	help
 //config:	The pivot_root utility swaps the mount points for the root filesystem
 //config:	with some other mounted filesystem. This allows you to do all sorts