Cmake build: Added code to compile with QT5.7

QT5.7 cmake config files explicitly instruct cmake to use `-std=gnu++11`.
This is not the same as is found by our FindCXX11Compiler.cmake,
which leads to compilation errors.
Here we overrule the setting.

Thanks to helpfull hand from Brad King <brad.king@kitware.com>

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 48688dced37a097cd8d5f6e31e62d09e64479551..7fd8b26f764b94a56aa9b8e7a1966548be3137a2 100644 (file)
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -572,7 +572,15 @@ if(NOT MSVC)
        if(NOT LYX_QUIET)
                set(CMAKE_VERBOSE_MAKEFILE ON)
        endif()
-    set(LYX_CXX_FLAGS "-Wall -Wunused-parameter ${LYX_GCC11_MODE}")
+       set(LYX_CXX_FLAGS "-Wall -Wunused-parameter ${LYX_GCC11_MODE}")
+       if(LYX_CXX_FLAGS MATCHES "\\+\\+(14|11|98)")
+         # Thanks to Brad King <brad.king@kitware.com>
+         # for the pointer to https://cmake.org/cmake/help/v3.6/variable/CMAKE_CXX_STANDARD.html
+         # This allows us to use QT5.7 with recent g++ (version >= 4.9) compilers
+         # and still use our own c++ extension tests
+         set(CMAKE_CXX_STANDARD ${CMAKE_MATCH_1})
+         message(STATUS "CMAKE_CXX_STANDARD set to ${CMAKE_CXX_STANDARD}")
+       endif()
        if(LYX_STDLIB_DEBUG)
                set(LYX_CXX_FLAGS "${LYX_CXX_FLAGS} -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC")
        endif()