LStream: Establishing C++ 14 by default for building without Qt6

src/plugins/streamers/lstream/db_plugin/db_plugin.pro

@@ -2,6 +2,7 @@
 TARGET = lstream
 DESTDIR = $$OUT_PWD/../../../../db_plugins
 
+include($$PWD/../lstream.pri)
 include($$PWD/../../../db_plugin.pri)
 include($$PWD/capnp/capnp.pri)
 

src/plugins/streamers/lstream/lay_plugin/lay_plugin.pro

@@ -1,6 +1,7 @@
 
 DESTDIR = $$OUT_PWD/../../../../lay_plugins
 
+include($$PWD/../lstream.pri)
 include($$PWD/../../../lay_plugin.pri)
 
 INCLUDEPATH += $$PWD/../db_plugin $$PWD/../db_plugin/capnp

src/plugins/streamers/lstream/lstream.pri

@@ -0,0 +1,15 @@
+
+# .c++ is not a standard C++ extension in qmake
+QMAKE_EXT_CPP += .c++
+
+# capnp needs C++ 14 in version 1.0.1
+# Qt6 comes with C++ 17 requirement.
+equals(HAVE_QT, "0") || lessThan(QT_MAJOR_VERSION, 6) {
+  QMAKE_CXXFLAGS += -std=c++14
+}
+
+!msvc {
+  # capnp runtimes have some unused arguments
+  QMAKE_CXXFLAGS += \
+      -Wno-unused-parameter
+}

src/plugins/streamers/lstream/runtime/capnp/capnp.pro

@@ -2,6 +2,7 @@
 TEMPLATE = lib
 DESTDIR = $$OUT_PWD/../../../../..
 
+include($$PWD/../../lstream.pri)
 include($$PWD/capnp.pri)
 
 TARGET = xcapnp

src/plugins/streamers/lstream/runtime/kj/kj.pro

@@ -2,6 +2,7 @@
 TEMPLATE = lib
 DESTDIR = $$OUT_PWD/../../../../..
 
+include($$PWD/../../lstream.pri)
 include($$PWD/kj.pri)
 
 TARGET = xkj

src/plugins/streamers/lstream/unit_tests/unit_tests.pro

@@ -3,6 +3,7 @@ DESTDIR_UT = $$OUT_PWD/../../../..
 
 TARGET = lstream_tests
 
+include($$PWD/../lstream.pri)
 include($$PWD/../../../../lib_ut.pri)
 
 SOURCES = \