qt54: set runtime env vars during build for makeWrapper

Set QT_PLUGIN_PATH, QML_IMPORT_PATH, and QML2_IMPORT_PATH during build
so that Qt programs may easily be wrapped for use outside NixOS. The
paths are set appropriately for the dependencies of the package.

pkgs/development/libraries/qt-5/5.4/qt-submodule.nix

···
   dontFixLibtool = args.dontFixLibtool or true;
   configureScript = args.configureScript or "qmake";
 
-  /*
-  preConfigure = ''
-    export PATH="$out/bin:$PATH"
-  '';
-  */
-
   postInstall = ''
     rm "$out/bin/qmake" "$out/bin/qt.conf"
 

pkgs/development/libraries/qt-5/5.4/setup-hook.sh

···
             if [[ -n $qtSubmodule ]]; then
                 find "$1/lib" -printf 'lib/%P\n' >> "$qtOut/nix-support/qt-inputs"
             fi
+
+            if [[ -d "$1/lib/qt5/plugins" ]]; then
+                QT_PLUGIN_PATH="$QT_PLUGIN_PATH${QT_PLUGIN_PATH:+:}$1/lib/qt5/plugins";
+            fi
+
+            if [[ -d "$1/lib/qt5/imports" ]]; then
+                QML_IMPORT_PATH="$QML_IMPORT_PATH${QML_IMPORT_PATH:+:}$1/lib/qt5/imports";
+            fi
+
+            if [[ -d "$1/lib/qt5/qml" ]]; then
+                QML2_IMPORT_PATH="$QML2_IMPORT_PATH${QML2_IMPORT_PATH:+:}$1/lib/qt5/qml";
+            fi
         fi
     fi
 }