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/qmakesettings.cpp

@@ -0,0 +1,141 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "qmakesettings.h"
+
+#include <coreplugin/icore.h>
+#include <projectexplorer/projectexplorerconstants.h>
+#include <utils/hostosinfo.h>
+
+#include <QCheckBox>
+#include <QCoreApplication>
+#include <QVBoxLayout>
+
+namespace QmakeProjectManager {
+namespace Internal {
+
+const char BUILD_DIR_WARNING_KEY[] = "QmakeProjectManager/WarnAgainstUnalignedBuildDir";
+
+static bool operator==(const QmakeSettingsData &s1, const QmakeSettingsData &s2)
+{
+    return s1.warnAgainstUnalignedBuildDir == s2.warnAgainstUnalignedBuildDir;
+}
+static bool operator!=(const QmakeSettingsData &s1, const QmakeSettingsData &s2)
+{
+    return !(s1 == s2);
+}
+
+bool QmakeSettings::warnAgainstUnalignedBuildDir()
+{
+    return instance().m_settings.warnAgainstUnalignedBuildDir;
+}
+
+QmakeSettings &QmakeSettings::instance()
+{
+    static QmakeSettings theSettings;
+    return theSettings;
+}
+
+void QmakeSettings::setSettingsData(const QmakeSettingsData &settings)
+{
+    if (instance().m_settings != settings) {
+        instance().m_settings = settings;
+        instance().storeSettings();
+        emit instance().settingsChanged();
+    }
+}
+
+QmakeSettings::QmakeSettings()
+{
+    loadSettings();
+}
+
+void QmakeSettings::loadSettings()
+{
+    m_settings.warnAgainstUnalignedBuildDir = Core::ICore::settings()->value(
+                BUILD_DIR_WARNING_KEY, Utils::HostOsInfo::isWindowsHost()).toBool();
+}
+
+void QmakeSettings::storeSettings() const
+{
+    Core::ICore::settings()->setValue(BUILD_DIR_WARNING_KEY, warnAgainstUnalignedBuildDir());
+}
+
+class QmakeSettingsPage::SettingsWidget : public QWidget
+{
+    Q_DECLARE_TR_FUNCTIONS(QmakeProjectManager::Internal::QmakeSettingsPage)
+public:
+    SettingsWidget()
+    {
+        m_warnAgainstUnalignedBuildDirCheckbox.setText(tr("Warn if a project's source and "
+            "build directories are not at the same level"));
+        m_warnAgainstUnalignedBuildDirCheckbox.setToolTip(tr("Qmake has subtle bugs that "
+            "can trigger if source and build directory are not at the same level."));
+        m_warnAgainstUnalignedBuildDirCheckbox.setChecked(
+                    QmakeSettings::warnAgainstUnalignedBuildDir());
+        const auto layout = new QVBoxLayout(this);
+        layout->addWidget(&m_warnAgainstUnalignedBuildDirCheckbox);
+        layout->addStretch(1);
+    }
+
+    void apply()
+    {
+        QmakeSettingsData settings;
+        settings.warnAgainstUnalignedBuildDir = m_warnAgainstUnalignedBuildDirCheckbox.isChecked();
+        QmakeSettings::setSettingsData(settings);
+    }
+
+private:
+    QCheckBox m_warnAgainstUnalignedBuildDirCheckbox;
+};
+
+QmakeSettingsPage::QmakeSettingsPage()
+{
+    setId("K.QmakeProjectManager.QmakeSettings");
+    setDisplayName(tr("Qmake"));
+    setCategory(ProjectExplorer::Constants::BUILD_AND_RUN_SETTINGS_CATEGORY);
+}
+
+QWidget *QmakeSettingsPage::widget()
+{
+    if (!m_widget)
+        m_widget = new SettingsWidget;
+    return m_widget;
+
+}
+
+void QmakeSettingsPage::apply()
+{
+    if (m_widget)
+        m_widget->apply();
+}
+
+void QmakeSettingsPage::finish()
+{
+    delete m_widget;
+}
+
+} // namespace Internal
+} // namespace QmakeProjectManager