ensure that there is always an -indexdir argument

even if we are not doing a top-level build, we still need to specify an
index dir. that may be the install dir or the qtbase build dir,
depending on whether we are building against an installed prefix build
or a non-prefix build (building against non-installed prefix builds
outside a top-level build is inherently impossible).

Task-number: QTBUG-35596
Change-Id: Ia37d429855480d3bfe36b7ee29e087029861bfc5
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>

mkspecs/features/qt_docs.prf

@@ -23,9 +23,13 @@ QDOC += -outputdir $$QMAKE_DOCS_OUTPUTDIR
 !build_online_docs: \
     QDOC += -installdir $$[QT_INSTALL_DOCS]
 DOC_INDEXES =
-for(qrep, QTREPOS): \
-    exists($$qrep/doc): \
-        DOC_INDEXES += -indexdir $$qrep/doc
+!isEmpty(QTREPOS) {
+    for(qrep, QTREPOS): \
+        exists($$qrep/doc): \
+            DOC_INDEXES += -indexdir $$qrep/doc
+} else {
+    DOC_INDEXES += -indexdir $$[QT_INSTALL_DOCS/get]
+}
 qtver.name = QT_VERSION
 qtver.value = $$VERSION
 isEmpty(qtver.value): qtver.value = $$MODULE_VERSION