ncurses: fix includedir setting

${out} in configureFlags isn't expanded, so ncursesw5-config ends up
expanding ${out} at *runtime*. Here is the relevant ncursesw5-config
snippet showing how includedir gets its value at runtime.

  bindir="${exec_prefix}/bin"
  includedir="${out}/include"
  libdir="${exec_prefix}/lib"
  datadir="${prefix}/share"
  mandir="${prefix}/man"

When running in a plain shell you get this:
  $ ncursesw5-config --cflags
  -I/include/ncursesw -I/include

And when run in a nix-build shell for e.g. gpsd:
  $ ncursesw5-config --cflags
  -I/nix/store/HASH-gpsd-3.10/include/ncursesw -I/nix/store/HASH-gpsd-3.10/include

This is clearly wrong.

Q: How come this has gone undetected for years?
A: It seems few packages use ncursesw5-config to get the compiler
flags. For example, our python curses module builds its own compiler
flags.

Fix this by moving the --includedir setting to preConfigure where shell
variables are expanded.

pkgs/development/libraries/ncurses/5_4.nix

@@ -19,10 +19,14 @@ stdenv.mkDerivation (rec {
   };
 
   configureFlags = ''
-    --with-shared --includedir=''${out}/include --without-debug
+    --with-shared --without-debug
     ${if unicode then "--enable-widec" else ""}${if cxx then "" else "--without-cxx-binding"}
   '';
 
+  preConfigure = ''
+    export configureFlags="$configureFlags --includedir=$out/include"
+  '';
+
   selfNativeBuildInput = true;
 
   enableParallelBuilding = true;

pkgs/development/libraries/ncurses/default.nix

@@ -18,12 +18,16 @@ stdenv.mkDerivation (rec {
     sha256 = "0fsn7xis81za62afan0vvm38bvgzg5wfmv1m86flqcj0nj7jjilh";
   };
 
+  patches = [ ./patch-ac ];
+
   configureFlags = ''
-    --with-shared --includedir=''${out}/include --without-debug
+    --with-shared --without-debug
     ${if unicode then "--enable-widec" else ""}${if cxx then "" else "--without-cxx-binding"}
   '';
 
-  patches = [ ./patch-ac ];
+  preConfigure = ''
+    export configureFlags="$configureFlags --includedir=$out/include"
+  '';
 
   selfNativeBuildInput = true;