Have configure.py modify internal variables depending on the --qml-plugindir
configure option.
diff -u PyQt-gpl-5.4.alt/configure.py PyQt-gpl-5.4/configure.py
--- PyQt-gpl-5.4.alt/configure.py	2015-01-25 17:27:50.000000000 +0100
+++ PyQt-gpl-5.4/configure.py	2015-01-25 17:56:41.000000000 +0100
@@ -904,6 +904,9 @@
         if opts.pyuicinterpreter is not None:
             self.pyuic_interpreter = opts.pyuicinterpreter
 
+        if opts.qmlplugindir is not None:
+            self.qml_plugin_dir = opts.qmlplugindir
+
         if opts.qsciapidir is not None:
             self.qsci_api_dir = opts.qsciapidir
 
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