strongswan: build with clang

Fixes https://lists.strongswan.org/pipermail/users/2014-September/006597.html

pkgs/tools/networking/strongswan/default.nix

···
     description = "OpenSource IPsec-based VPN Solution";
     homepage = https://www.strongswan.org;
     license = stdenv.lib.licenses.gpl2Plus;
+    inherit (stdenv.gcc.clang.meta) platforms;
   };
 }

pkgs/top-level/all-packages.nix

···
 
   stunnel = callPackage ../tools/networking/stunnel { };
 
-  strongswan = callPackage ../tools/networking/strongswan { };
+  strongswan = callPackage ../tools/networking/strongswan { stdenv = clangStdenv; };
 
   su = shadow.su;