darwin.libiconv: use static library

This is necessary to get stuff into the Apple App Store for iOS. Apple does not really like using dynamic libraries in iOS. Also the libcharset thing is apparently a "private" symbol.

See https://github.com/reflex-frp/reflex-platform/pull/307 for more details.

See 3dee596ed1 (diff-261d74054a31954b7c6175e2e7e28356) for discussion

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

@@ -1,4 +1,4 @@
-{ stdenv, appleDerivation, autoreconfHook }:
+{ stdenv, appleDerivation, autoreconfHook, targetPlatform, enableStatic ? targetPlatform.isiOS }:
 
 appleDerivation {
   postUnpack = "sourceRoot=$sourceRoot/libiconv";
@@ -7,7 +7,9 @@ appleDerivation {
     sed -i 's/darwin\*/ios\*/g' configure libcharset/configure
   '';
 
-  postInstall = ''
+  configureFlags = stdenv.lib.optionals enableStatic [ "--enable-static" "--disable-shared" ];
+
+  postInstall = stdenv.lib.optionalString (!enableStatic) ''
     mv $out/lib/libiconv.dylib $out/lib/libiconv-nocharset.dylib
     ${stdenv.cc.bintools.targetPrefix}install_name_tool -id $out/lib/libiconv-nocharset.dylib $out/lib/libiconv-nocharset.dylib