Fix build after const removal from iconv.

Reported by:	ajtiM <lumiwa at gmail.com> (via email)

games/0ad/files/patch-build__premake__extern_libs4.lua

@@ -1,6 +1,6 @@
---- build/premake/extern_libs4.lua.orig	2014-09-24 10:05:27 UTC
+--- build/premake/extern_libs4.lua.orig	2015-02-20 05:52:18 UTC
 +++ build/premake/extern_libs4.lua
-@@ -237,7 +237,7 @@
+@@ -237,7 +237,7 @@ extern_lib_defs = {
  			end
  			-- TODO: This actually applies to most libraries we use on BSDs, make this a global setting.
  			if os.is("bsd") then
@@ -9,3 +9,12 @@
  			end
  		end,
  		link_settings = function()
+@@ -346,8 +346,6 @@ extern_lib_defs = {
+ 			elseif os.is("macosx") then
+ 				add_default_include_paths("iconv")
+ 				defines { "LIBICONV_STATIC" }
+-			elseif os.getversion().description == "FreeBSD" then
+-				defines { "HAVE_ICONV_CONST" }
+ 			end
+ 		end,
+ 		link_settings = function()