Bump C++ version to 14

Our compilers support some features which are part of the C++ 14 feature
set. Some off them are silently compiled with the C++ 11 setting.

For example:

[foo=bar] {}

is actually working with GCC but is part of C++ 14.

Change-Id: Icf2c3806e26c675f8251768c6ad54ba4da186d11
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

.qmake.conf

@@ -1 +1 @@
-CONFIG += c++11
+CONFIG += c++14

qbs/imports/QtcProduct.qbs

@@ -19,7 +19,7 @@ Product {
     Depends { name: "qtc" }
     Depends { name: product.name + " dev headers"; required: false }
 
-    cpp.cxxLanguageVersion: "c++11"
+    cpp.cxxLanguageVersion: "c++14"
     cpp.defines: qtc.generalDefines
     cpp.minimumOsxVersion: "10.7"
     cpp.minimumWindowsVersion: qbs.architecture === "x86" ? "5.1" : "5.2"