ncurses5: Use v6 sources but compiled with --with-abi-version=5

So we don't have to dig out why ncurses5.9 stops compiling with newer
compiler versions again and again.

Inspired by:

https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=ncurses5-compat-libs

pkgs/development/libraries/ncurses/default.nix

@@ -11,23 +11,18 @@
 }:
 
 stdenv.mkDerivation rec {
-  version = if abiVersion == "5" then "5.9" else "6.0-20171125";
-  name = "ncurses-${version}";
+  version = "6.0-20171125";
+  name = "ncurses-${version}" + lib.optionalString (abiVersion == "5") "-abi5-compat";
 
-  src = fetchurl (if abiVersion == "5" then {
-    url = "mirror://gnu/ncurses/${name}.tar.gz";
-    sha256 = "0fsn7xis81za62afan0vvm38bvgzg5wfmv1m86flqcj0nj7jjilh";
-  } else {
+  src = fetchurl {
     urls = [
-      "ftp://ftp.invisible-island.net/ncurses/current/${name}.tgz"
-      "https://invisible-mirror.net/archives/ncurses/current/${name}.tgz"
+      "ftp://ftp.invisible-island.net/ncurses/current/ncurses-${version}.tgz"
+      "https://invisible-mirror.net/archives/ncurses/current/ncurses-${version}.tgz"
     ];
     sha256 = "11adzj0k82nlgpfrflabvqn2m7fmhp2y6pd7ivmapynxqb9vvb92";
-  });
+  };
 
-  # Unnecessarily complicated in order to avoid mass-rebuilds
-  patches = lib.optional (!stdenv.cc.isClang || abiVersion == "5") ./clang.patch
-    ++ lib.optional (stdenv.cc.isGNU && abiVersion == "5") ./gcc-5.patch;
+  patches = lib.optional (!stdenv.cc.isClang) ./clang.patch;
 
   outputs = [ "out" "dev" "man" ];
   setOutputFlags = false; # some aren't supported
@@ -37,7 +32,8 @@ stdenv.mkDerivation rec {
     "--without-debug"
     "--enable-pc-files"
     "--enable-symlinks"
-  ] ++ lib.optional unicode "--enable-widec";
+  ] ++ lib.optional unicode "--enable-widec"
+    ++ lib.optional (abiVersion == "5") "--with-abi-version=5";
 
   # Only the C compiler, and explicitly not C++ compiler needs this flag on solaris:
   CFLAGS = lib.optionalString stdenv.isSunOS "-D_XOPEN_SOURCE_EXTENDED";

pkgs/top-level/all-packages.nix

@@ -10297,7 +10297,6 @@ with pkgs;
   notify-sharp = callPackage ../development/libraries/notify-sharp { };
 
   ncurses5 = callPackage ../development/libraries/ncurses {
-    stdenv = if stdenv.cc.isGNU then overrideCC stdenv gcc6 else stdenv; # Hacky!
     abiVersion = "5";
   };
   ncurses6 = callPackage ../development/libraries/ncurses {