ncurses: simplify patch expr, fix ncurses6 build w/clang

Somewhat amusingly given its name, "clang.patch" applies to both 5 and 6
but is the cause of ncurses6 breakage on 6 but is required on 5...
gcc is happy in all four configurations:

       5  5p 6  6p
gcc    ✓  ✓  ✓  ✓
clang  ✗  ✓  ✓  ✗

Which is why this commit enables the patch for 5 but not 6;
this matches behavior in Gentoo, for example.

For further simplification, we also use gcc-5 patch regardless.

pkgs/development/libraries/ncurses/default.nix

@@ -22,7 +22,7 @@ stdenv.mkDerivation rec {
     sha256 = "11adzj0k82nlgpfrflabvqn2m7fmhp2y6pd7ivmapynxqb9vvb92";
   });
 
-  patches = [ ./clang.patch ] ++ lib.optional (abiVersion == "5" && stdenv.cc.isGNU) ./gcc-5.patch;
+  patches = lib.optionals (abiVersion == "5") [ ./clang.patch ./gcc-5.patch ];
 
   outputs = [ "out" "dev" "man" ];
   setOutputFlags = false; # some aren't supported