make force_independent live up to its name

don't install the module .pri file into qtbase even when doing a
non-prefix build.

this has no effect on modules built as part of a top-level build, as
they announce themselves via .qmake.super anyway.
however, modules built separately become unavailable unless QMAKEPATH
or QMAKEMODULES is set. this is deemed not relevant by the original
audience of this feature (the qtwebkit team).

Change-Id: I14c170b2c5dbb99608939aef1a541563d5b755d9
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

mkspecs/features/qt_build_paths.prf

@@ -20,10 +20,9 @@ isEmpty(MODULE_QMAKE_OUTDIR): MODULE_QMAKE_OUTDIR = $$MODULE_BASE_OUTDIR
 exists($$MODULE_BASE_INDIR/.git): \
     CONFIG += git_build
 
-!prefix_build {
-    QTDIR = $$[QT_HOST_PREFIX]
-    # Permit modules to enforce being built outside QTDIR ...
-    !force_independent: MODULE_BASE_OUTDIR = $$QTDIR
-    # ... though this sort of breaks the idea.
-    MODULE_QMAKE_OUTDIR = $$QTDIR
+!force_independent {
+    # If the module is not built independently, everything ends up in qtbase.
+    # This is the case in non-prefix builds, except for selected modules.
+    MODULE_BASE_OUTDIR = $$[QT_HOST_PREFIX]
+    MODULE_QMAKE_OUTDIR = $$[QT_HOST_PREFIX]
 }