enumerate modules in QT_MODULES, not QT_CONFIG

QT_CONFIG is supposed to contain configure output, not a list of
modules. for example, enumerating modules is not cleanly possible if
modules are mixed with other flags.

the conflation was merely historical, due to webkit and phonon doing
it this way in the preliminary qt4 modularization.

we now have a much cleaner way to query modules (qtHaveModule(<module>),
or less recently, !isEmpty(QT.<module>.name)), which is already used
throughout Qt.

the old way was supposed to be removed for 5.0 already, but it slipped.
better do it now, before people actually start using it.

Change-Id: Iabdf0cdfaab9cd674f634f4c6ece105b2039c850
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

mkspecs/features/qt_module_pris.prf

@@ -71,7 +71,7 @@ else: \
         $$module_config \
         "QT.$${MODULE}.DEFINES = $$MODULE_DEFINES" \ # assume sufficient quoting
         "" \
-        "QT_CONFIG += $$MODULE" # this is obsolete, but some code still depends on it
+        "QT_MODULES += $$MODULE"
     write_file($$MODULE_PRI, MODULE_PRI_CONT)|error("Aborting.")
     MODULE_PRI_FILES = $$MODULE_PRI
 
@@ -113,7 +113,7 @@ else: \
                     includes private_includes bins libs libexecs plugins imports qml \
                     rpath_link rpath_link_private \
             )):defined(QT.$${MODULE}.$$var, var):cache(QT.$${MODULE}.$$var, transient)
-    cache(QT_CONFIG, transient)
+    cache(QT_MODULES, transient)
 
 } # !build_pass