QmakePM: Only consider the first make step for subproject builds

When the user requests to build a sub-project or a single file of a
qmake project, we simply run the normal steps from the build
configuration, and the make steps modify themselves to run in the
respective subdirectory and/or with specific targets. Normally, there is
only one such make step. If the user added more make steps, it is very
likely that these only make sense for a top-level build, so we simply
make them no-ops when building a sub-project or single file.

Fixes: QTCREATORBUG-15794
Change-Id: I960cf60dba5e5d6b6e839d35e34650427f659110
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/plugins/qmakeprojectmanager/qmakemakestep.cpp

@@ -83,6 +83,10 @@ bool QmakeMakeStep::init()
         return false;
     }
 
+    // Ignore all but the first make step for a non-top-level build. See QTCREATORBUG-15794.
+    m_ignoredNonTopLevelBuild = (bc->fileNodeBuild() || bc->subNodeBuild())
+            && static_cast<BuildStepList *>(parent())->firstOfType<QmakeMakeStep>() != this;
+
     ProcessParameters *pp = processParameters();
     pp->setMacroExpander(bc->macroExpander());
 
@@ -177,7 +181,7 @@ bool QmakeMakeStep::init()
 
 void QmakeMakeStep::doRun()
 {
-    if (m_scriptTarget) {
+    if (m_scriptTarget || m_ignoredNonTopLevelBuild) {
         emit finished(true);
         return;
     }

src/plugins/qmakeprojectmanager/qmakemakestep.h

@@ -62,6 +62,7 @@ private:
     bool m_scriptTarget = false;
     QString m_makeFileToCheck;
     bool m_unalignedBuildDir;
+    bool m_ignoredNonTopLevelBuild = false;
 };
 
 } // QmakeProjectManager