Workaround bug where documentation sometimes is not generated.

On Windows, for unknown reasons, qhelpgenerator.exe sometimes will
not generate the documentation if run directly from nmake.
Running it through an intermediate cmd seems to work.
So, do that.

Reviewed-by: Daniel Molkentin

doc/doc.pri

@@ -19,8 +19,10 @@ unix {
     QDOC = SRCDIR=$$PWD OUTDIR=$$OUT_PWD/doc/html $$QDOC_BIN 
     HELPGENERATOR = $$(QTDIR)/bin/qhelpgenerator
 } else {
-    QDOC = set SRCDIR=$$PWD&& set OUTDIR=$$OUT_PWD/doc/html&& $$QDOC_BIN 
-    HELPGENERATOR = $$(QTDIR)\bin\qhelpgenerator.exe
+    QDOC = set SRCDIR=$$PWD&& set OUTDIR=$$OUT_PWD/doc/html&& $$QDOC_BIN
+    # Always run qhelpgenerator inside its own cmd; this is a workaround for
+    # an unusual bug which causes qhelpgenerator.exe to do nothing
+    HELPGENERATOR = cmd /C $$(QTDIR)\bin\qhelpgenerator.exe
 }
 
 QHP_FILE = $$OUT_PWD/doc/html/qtcreator.qhp