Qmake: Improve the build dir location warning

The infamous "build dir is not at the same level at source dir" warning
for qmake projects has gone through a number of iterations in Qt
Creator, having been added, removed and re-added, always to the
criticism of some users. Our new approach is as follows:
    - The warning appears at the widgets where the build directory is
set, both in the target setup page and the build config widget.
    - The warning also appears in the issues pane, but only if the build
failed.
    - The user can disable the warning altogether in a newly introduced
qmake settings page.
    - This option is disabled by default on Unix, because to my
knowledge all failure reports have been for Windows hosts.
This should finally please everybody.

Fixes: QTCREATORBUG-16945
Change-Id: I638be1f15e8c260a5d72047d6850a3a0f685cf03
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/plugins/qmakeprojectmanager/qmakebuildconfiguration.cpp

@@ -31,6 +31,7 @@
 #include "qmakeprojectconfigwidget.h"
 #include "qmakeprojectmanagerconstants.h"
 #include "qmakenodes.h"
+#include "qmakesettings.h"
 #include "qmakestep.h"
 #include "qmakemakestep.h"
 #include "makefileparse.h"
@@ -293,6 +294,23 @@ void QmakeBuildConfiguration::emitProFileEvaluateNeeded()
         static_cast<QmakeProject *>(p)->scheduleAsyncUpdate();
 }
 
+QString QmakeBuildConfiguration::unalignedBuildDirWarning()
+{
+    return tr("The build directory should be at the same level as the source directory.");
+}
+
+bool QmakeBuildConfiguration::isBuildDirAtSafeLocation(const QString &sourceDir,
+                                                       const QString &buildDir)
+{
+    return buildDir.count('/') == sourceDir.count('/');
+}
+
+bool QmakeBuildConfiguration::isBuildDirAtSafeLocation() const
+{
+    return isBuildDirAtSafeLocation(project()->projectDirectory().toString(),
+                                    buildDirectory().toString());
+}
+
 void QmakeBuildConfiguration::emitQMakeBuildConfigurationChanged()
 {
     emit qmakeBuildConfigurationChanged();
@@ -571,19 +589,11 @@ QmakeBuildConfigurationFactory::QmakeBuildConfigurationFactory()
         QList<Task> issues;
         if (version)
             issues << version->reportIssues(projectPath, buildDir);
-
-        QString tmpBuildDir = QDir(buildDir).absolutePath();
-        const QChar slash = QLatin1Char('/');
-        if (!tmpBuildDir.endsWith(slash))
-            tmpBuildDir.append(slash);
-        QString sourcePath = QFileInfo(projectPath).absolutePath();
-        if (!sourcePath.endsWith(slash))
-            sourcePath.append(slash);
-        if (tmpBuildDir.count(slash) != sourcePath.count(slash)) {
-            const QString msg = QCoreApplication::translate("QmakeProjectManager::QtVersion",
-                                                            "The build directory needs to be at the same level as the source directory.");
-
-            issues.append(Task(Task::Warning, msg, Utils::FileName(), -1,
+        if (QmakeSettings::warnAgainstUnalignedBuildDir()
+                && !QmakeBuildConfiguration::isBuildDirAtSafeLocation(
+                    QDir(projectPath).absolutePath(), QDir(buildDir).absolutePath())) {
+            issues.append(Task(Task::Warning, QmakeBuildConfiguration::unalignedBuildDirWarning(),
+                               Utils::FileName(), -1,
                                ProjectExplorer::Constants::TASK_CATEGORY_BUILDSYSTEM));
         }
         return issues;