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/dmesg.c b/util-linux/dmesg.c
index 6670b84..5da887f 100644
--- a/util-linux/dmesg.c
+++ b/util-linux/dmesg.c
@@ -11,6 +11,7 @@
 //config:config DMESG
 //config:	bool "dmesg (3.7 kb)"
 //config:	default y
+//config:	select PLATFORM_LINUX
 //config:	help
 //config:	dmesg is used to examine or control the kernel ring buffer. When the
 //config:	Linux kernel prints messages to the system log, they are stored in