libiconv: use enableFeature for static/shared

This makes things more predictable w.r.t. what is actually being built.

pkgs/development/libraries/libiconv/default.nix

@@ -29,9 +29,10 @@ stdenv.mkDerivation rec {
       sed -i -e '/preload/d' Makefile.in
     '';
 
-  configureFlags = lib.optional stdenv.isFreeBSD "--with-pic"
-    ++ lib.optional enableStatic "--enable-static"
-    ++ lib.optional (!enableShared) "--disable-shared";
+  configureFlags = [
+    (lib.enableFeature enableStatic "static")
+    (lib.enableFeature enableShared "shared")
+  ] ++ lib.optional stdenv.isFreeBSD "--with-pic";
 
   meta = {
     description = "An iconv(3) implementation";

pkgs/os-specific/darwin/apple-source-releases/libiconv/default.nix

@@ -10,8 +10,10 @@ appleDerivation {
     sed -i 's/darwin\*/ios\*/g' configure libcharset/configure
   '';
 
-  configureFlags = lib.optional enableStatic "--enable-static"
-                ++ lib.optional (!enableShared) "--disable-shared";
+  configureFlags = [
+    (lib.enableFeature enableStatic "static")
+    (lib.enableFeature enableShared "shared")
+  ];
 
   postInstall = lib.optionalString (!enableStatic) ''
     mv $out/lib/libiconv.dylib $out/lib/libiconv-nocharset.dylib