qmake: Select debug or release build

The qmake hook sets its own `CONFIG+=debug` or `CONFIG+=release` depending on
how `qtbase` was built. We no longer rely on using the custom deriver for this
feature.

pkgs/development/libraries/qt-5/hooks/qmake-hook.sh

@@ -3,6 +3,9 @@
 qmakeFlags=( ${qmakeFlags-} )
 
 qmakePrePhase() {
+    qmakeFlags_orig=( "${qmakeFlags[@]}" )
+
+    # These flags must be added _before_ the flags specified in the derivation.
     qmakeFlags=( \
         "PREFIX=$out" \
         "NIX_OUTPUT_OUT=$out" \
@@ -11,8 +14,15 @@ qmakePrePhase() {
         "NIX_OUTPUT_DOC=${!outputDev}/${qtDocPrefix:?}" \
         "NIX_OUTPUT_QML=${!outputBin}/${qtQmlPrefix:?}" \
         "NIX_OUTPUT_PLUGIN=${!outputBin}/${qtPluginPrefix:?}" \
-        "${qmakeFlags[@]}" \
     )
+
+    if [ -n "@debug@" ]; then
+        qmakeFlags+=( "CONFIG+=debug" )
+    else
+        qmakeFlags+=( "CONFIG+=release" )
+    fi
+
+    qmakeFlags+=( "${qmakeFlags_orig[@]}" )
 }
 prePhases+=" qmakePrePhase"