gmp-*: Clean up configureFlags

pkgs/development/libraries/gmp/6.x.nix

@@ -21,18 +21,17 @@ let self = stdenv.mkDerivation rec {
   depsBuildBuild = [ buildPackages.stdenv.cc ];
   nativeBuildInputs = [ m4 ];
 
-  configureFlags =
+  configureFlags = [
+    "--with-pic"
+    (stdenv.lib.enableFeature cxx "cxx")
     # Build a "fat binary", with routines for several sub-architectures
     # (x86), except on Solaris where some tests crash with "Memory fault".
     # See <http://hydra.nixos.org/build/2760931>, for instance.
     #
     # no darwin because gmp uses ASM that clang doesn't like
-    optional (!stdenv.isSunOS && stdenv.hostPlatform.isx86) "--enable-fat"
-    ++ (if cxx then [ "--enable-cxx"  ]
-               else [ "--disable-cxx" ])
-    ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
+    (stdenv.lib.enableFeature (!stdenv.isSunOS && stdenv.hostPlatform.isx86) "fat")
+  ] ++ optional (cxx && stdenv.isDarwin) "CPPFLAGS=-fexceptions"
     ++ optional (stdenv.isDarwin && stdenv.is64bit) "ABI=64"
-    ++ optional stdenv.is64bit "--with-pic"
     ++ optional (with stdenv.hostPlatform; useAndroidPrebuilt || useiOSPrebuilt) "--disable-assembly"
     ;