C++14 standard as a build requirement

We're using some C++14 and later functionalities.

Change-Id: Ifb0dbf401c084b57312c5744684faf4d8b932db7
Reviewed-by: Nicolas Jäger <nicolas.jager@savoirfairelinux.com>

CMakeLists.txt

@@ -20,7 +20,6 @@ INCLUDE(GNUInstallDirs)
 INCLUDE(CMakePackageConfigHelpers)
 INCLUDE(GenerateExportHeader)
 
-ADD_DEFINITIONS("-std=c++1y")
 ADD_DEFINITIONS(
    ${QT_DEFINITIONS}
    -fexceptions
@@ -40,6 +39,9 @@ IF (CMAKE_COMPILER_IS_GNUCC)
    ENDIF()
 ENDIF()
 
+SET(CMAKE_CXX_STANDARD 14)
+SET(CXX_STANDARD_REQUIRED TRUE)
+
 SET(CMAKE_AUTOMOC TRUE)
 
 SET(LOCAL_CMAKE_MODULE_PATH "${CMAKE_SOURCE_DIR}/cmake/")