qmake build: Do not require 3rd party plugins to set c++14

Do it in qtcreator.pri instead again, since that is supposed to be
included by projects depending on Qt Creator.
Since we will use C++14 in utils/algorithm.h and other prominent
places, it would be very annoying to require setting C++14 on
all external plugins.

Change-Id: Ie11a30b33c791f3bf6db39fcee454f1e887352c2
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

.qmake.conf

-CONFIG += c++14

qtcreator.pri

@@ -6,6 +6,8 @@ QTCREATOR_COMPAT_VERSION = 4.2.82
 VERSION = $$QTCREATOR_VERSION
 BINARY_ARTIFACTS_BRANCH = master
 
+CONFIG += c++14
+
 defineReplace(qtLibraryTargetName) {
    unset(LIBRARY_NAME)
    LIBRARY_NAME = $$1

tests/unit/echoserver/echoserver.pro

@@ -10,6 +10,7 @@ unix:LIBS += -ldl
 
 osx:QMAKE_CXXFLAGS = -stdlib=libc++
 
+include(../../../qtcreator.pri)
 include(../../../src/libs/clangbackendipc/clangbackendipc-lib.pri)
 include(../../../src/libs/sqlite/sqlite-lib.pri)