[LyX/2.3.x] Next deprecation fix backport

[Juergen Spitzmueller]

commit d53e74d11b95c98de385fb332b228fe80e9e62dc
Author: Juergen Spitzmueller <spitz at lyx.org>
Date:   Sat Mar 13 10:47:53 2021 +0100

    Next deprecation fix backport
---
 src/frontends/qt4/GuiAbout.cpp |   28 +++++++++++++++++++++++-----
 1 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/src/frontends/qt4/GuiAbout.cpp b/src/frontends/qt4/GuiAbout.cpp
index 72b67e7..72a6306 100644
--- a/src/frontends/qt4/GuiAbout.cpp
+++ b/src/frontends/qt4/GuiAbout.cpp
@@ -240,13 +240,31 @@ static QString buildinfo()
 {
 	QString res;
 	QTextStream out(&res);
-	out << "LyX " << lyx_version
-		<< " (" << lyx_release_date << ")" << endl;
+	out << "LyX "
+	    << lyx_version
+	    << " (" 
+	    << lyx_release_date 
+	    << ")" 
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
+	    << Qt::endl;
+#else
+	    << endl;
+#endif
 	if (std::string(lyx_git_commit_hash) != "none")
 		out << qt_("  Git commit hash ")
-		    << QString(lyx_git_commit_hash).left(8) << endl;
-
-	out << lyx_version_info << endl;
+		    << QString(lyx_git_commit_hash).left(8)
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
+	    	    << Qt::endl;
+#else
+	    	    << endl;
+#endif
+
+	out << lyx_version_info
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 15, 0))
+	    << Qt::endl;
+#else
+	    << endl;
+#endif
 	return res;
 }