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/freeramdisk.c b/util-linux/freeramdisk.c
index 309169d..d27113d 100644
--- a/util-linux/freeramdisk.c
+++ b/util-linux/freeramdisk.c
@@ -11,6 +11,7 @@
 //config:config FDFLUSH
 //config:	bool "fdflush (1.3 kb)"
 //config:	default y
+//config:	select PLATFORM_LINUX
 //config:	help
 //config:	fdflush is only needed when changing media on slightly-broken
 //config:	removable media drives. It is used to make Linux believe that a
@@ -23,6 +24,7 @@
 //config:config FREERAMDISK
 //config:	bool "freeramdisk (1.3 kb)"
 //config:	default y
+//config:	select PLATFORM_LINUX
 //config:	help
 //config:	Linux allows you to create ramdisks. This utility allows you to
 //config:	delete them and completely free all memory that was used for the