OSX: change CXXFLAGS_CXX11 to be used not only with clang

Otherwise, zillions of warnings are displayed like:
  "defaulted and deleted functions only available with -std=c++11 or -std=gnu++11"
  "override controls (override/final) only available with -std=c++11 or -std=gnu++11"

Change-Id: Icedaf55c4f301d44587141ad937abbfce8d10457
Reviewed-on: https://gerrit.libreoffice.org/10130
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>