ncurses: header location fixes

- move headers directly to $out/include and set up symlinks.
Some packages were failing to find them (e.g. mariadb).
- postInstall was failing, only it was ignored due to a bug;
now it succeeds.

pkgs/development/libraries/ncurses/default.nix

@@ -54,6 +54,8 @@ stdenv.mkDerivation rec {
     ln -svf $cfg $out/bin/ncurses-config
 
     # Allow for end users who #include <ncurses?w/*.h>
+    mv "$out"/include/ncursesw/* "$out"/include/
+    rmdir "$out"/include/ncursesw
     ln -svf . $out/include/ncursesw
     ln -svf . $out/include/ncurses
 
@@ -83,6 +85,8 @@ stdenv.mkDerivation rec {
     ln -svf $cfg $out/bin/ncurses-config
 
     # Allow for end users who #include <ncurses/*.h>
+    mv "$out"/include/ncurses/* "$out"/include/
+    rmdir "$out"/include/ncurses
     ln -svf . $out/include/ncurses
 
     # Create curses compatability