libnl: do not allow to build tools on static lib configs

Building the libnl tools requires the <dlfcn.h> header, which is not
compatible with static only builds. Therefore, this commit makes the
libnl tools option depend on !BR2_PREFER_STATIC_LIB.

Fixes:

   http://autobuild.buildroot.org/results/573/57340b9bd2db3a523836e02e01cba6f7c17c8ca7/build-end.log

[Peter: fix tools comment dependency]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>

package/libnl/Config.in

@@ -6,11 +6,19 @@ config BR2_PACKAGE_LIBNL
 
 	  http://people.suug.ch/~tgr/libnl/
 
+comment "libnl needs a toolchain w/ threads"
+	depends on !BR2_TOOLCHAIN_HAS_THREADS
+
+if BR2_PACKAGE_LIBNL
+
 config BR2_PACKAGE_LIBNL_TOOLS
 	bool "install tools"
-	depends on BR2_PACKAGE_LIBNL
+	# uses <dlfcn.h>
+	depends on !BR2_PREFER_STATIC_LIB
 	help
 	  Install binary tools.
 
-comment "libnl needs a toolchain w/ threads"
-	depends on !BR2_TOOLCHAIN_HAS_THREADS
+comment "libnl tools need a toolchain w/ dynamic library"
+	depends on BR2_PREFER_STATIC_LIB
+
+endif