ncurses: install *.pc files (for pkg-config)

To make e.g. "pkg-config --cflags ncursesw" work.

The ncurses expression, when built in unicode/wide char mode, provides
backward compatibility symlinks from lib<name>.so to lib<name>w.so.
Provide similar symlinks for the *.pc files: <name>.pc -> <name>w.pc.

pkgs/development/libraries/ncurses/default.nix

@@ -21,12 +21,20 @@ stdenv.mkDerivation (rec {
   patches = [ ./patch-ac ];
 
   configureFlags = ''
-    --with-shared --without-debug
+    --with-shared --without-debug --enable-pc-files
     ${if unicode then "--enable-widec" else ""}${if cxx then "" else "--without-cxx-binding"}
   '';
 
+  # PKG_CONFIG_LIBDIR is where the *.pc files will be installed. If this
+  # directory doesn't exist, the configure script will disable installation of
+  # *.pc files. The configure script usually (on LSB distros) pick $(path of
+  # pkg-config)/../lib/pkgconfig. On NixOS that path doesn't exist and is not
+  # the place we want to put *.pc files from other packages anyway. So we must
+  # tell it explicitly where to install with PKG_CONFIG_LIBDIR.
   preConfigure = ''
     export configureFlags="$configureFlags --includedir=$out/include"
+    export PKG_CONFIG_LIBDIR="$out/lib/pkgconfig"
+    mkdir -p "$PKG_CONFIG_LIBDIR"
   '';
 
   selfNativeBuildInput = true;
@@ -50,6 +58,7 @@ stdenv.mkDerivation (rec {
         echo "INPUT(-l''${lib}w)" > $out/lib/lib$lib.so
         ln -svf lib''${lib}w.a $out/lib/lib$lib.a
         ln -svf lib''${lib}w.so.5 $out/lib/lib$lib.so.5
+        ln -svf ''${lib}w.pc $out/lib/pkgconfig/$lib.pc
       fi
     done;
     ln -svf . $out/include/ncursesw