[LyX/master] Make it possible to select C++ standard with autoconf

[Pavel Sanda]

On Tue, Nov 03, 2020 at 02:44:43PM +0100, Jean-Marc Lasgouttes wrote:
> commit ce526607ff4ae33834dabe64e0072530eca16183
> Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
> Date:   Mon Nov 2 17:40:29 2020 +0100
> 
>     Make it possible to select C++ standard with autoconf
>     
>     Introduce new configure option --enable-cxx-mode=MODE, which allows to
>     force a C++ version. The default is {14,11}, which means that C++14 is
>     chosen if it is supported, and C++11 will be selected as a fallback.
>     
>     Using --enable-cxx-mode=11 ensures that LyX compiles correctly
>     with an older C++11 compiler.

Is this supposed to work by default on old compilers or do I need to 
pass cxx-mode now?

checking for a good C++ mode... none
configure: error: Cannot find suitable mode for compiler g++
make: *** [Makefile:502: config.status] Error 1

Pavel