strongswan: unavailable for static builds

Uses dlopen in plugin_loader (not optional), fixes:
http://autobuild.buildroot.net/results/601/601d8dc1654d8733db49b195139e12437663034c/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/strongswan/Config.in

@@ -1,11 +1,12 @@
-comment "strongswan needs a toolchain w/ threads"
+comment "strongswan needs a toolchain w/ threads, dynamic library"
 	depends on BR2_USE_MMU
-	depends on !BR2_TOOLCHAIN_HAS_THREADS
+	depends on !BR2_TOOLCHAIN_HAS_THREADS || BR2_STATIC_LIBS
 
 menuconfig BR2_PACKAGE_STRONGSWAN
 	bool "strongswan"
 	depends on BR2_USE_MMU # fork()
 	depends on BR2_TOOLCHAIN_HAS_THREADS
+	depends on !BR2_STATIC_LIBS
 	help
 	  strongSwan is an OpenSource IPsec implementation for the
 	  Linux operating system. It is based on the discontinued