QmakeProjectManager: Fix missing run button activation

The problem could be reproduced as follows:
  - open any bigger qmake based project (e.g. QtCreator)
  - de-activate all kits
  - activate one kit
  - wait until the first parse (e.g. Debug build finished)
  - switch to Profile, and toggle the Shadow build button

The toggling re-starts the parsing.  The parse guard will not be released
in decrementPendingEvaluateFutures() (arguably correct as the parsing as
a whole is not done). On enteringing incrementPendingEvaluateFuture()
due to the re-start, the guard still guards the parse, but a new guard
is created in guardParsingRun(). As this happens while the build system
isParsing() is true, the created parse guard will be "non-guarding".
On assigning to m_guard, the original guard object will fire and then
be replaced by the non-guarding guard, that never fires again, so the
the build system will never have hasParsingData(), which is a
condition for the run buttons to activate.

As a workaround for people running into this issue in 4.12.0:
Select 'Run qmake' from the menu (any subproject suffices), and let
it finish.

Change-Id: If7a55db56ec67bac2635fb9a745b9aaf6ca6a413
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/qmakeprojectmanager/qmakeproject.cpp

@@ -500,8 +500,13 @@ void QmakeBuildSystem::startAsyncTimer(QmakeProFile::AsyncUpdateDelay delay)
 
 void QmakeBuildSystem::incrementPendingEvaluateFutures()
 {
-    if (m_pendingEvaluateFuturesCount == 0)
+    if (m_pendingEvaluateFuturesCount == 0) {
+        // The guard actually might already guard the project if this
+        // here is the re-start of a previously aborted parse due to e.g.
+        // changing build directories while parsing.
+        if (!m_guard.guardsProject())
             m_guard = guardParsingRun();
+    }
     ++m_pendingEvaluateFuturesCount;
     m_asyncUpdateFutureInterface.setProgressRange(m_asyncUpdateFutureInterface.progressMinimum(),
                                                   m_asyncUpdateFutureInterface.progressMaximum() + 1);