build: Require C++11
build: Require C++11

This was already required in practice, as the mozjs24 headers included
C++11 features. Probably this was not noticed because GCC did not warn
about that, but Clang does.

https://bugzilla.gnome.org/show_bug.cgi?id=742249

endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs,endlessm/gjs