Enable doc generation on non-windows platforms

This change enables running 'make docs' on platforms other than
Windows.

Change-Id: Ia6a03161db99fbe054f11e19f7cf152aa56574fb
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/src.pro

-TEMPLATE = subdirs
-CONFIG += ordered
-win32:SUBDIRS += winextras imports
+win32 {
+    TEMPLATE = subdirs
+    CONFIG += ordered
+    SUBDIRS += winextras imports
+} else {
+    # fake project for creating the documentation
+    TEMPLATE = aux
+    QMAKE_DOCS = $$PWD/winextras/doc/qtwinextras.qdocconf
+}