[LyX/master] Gcc 6+ use C++14 as default, so there is no need to enforce C++11

[Jean-Marc Lasgouttes]

commit 67ac031a33b1621fdadb903422de598595cc08d2
Author: Jean-Marc Lasgouttes <lasgout...@lyx.org>
Date:   Thu Jun 30 11:15:14 2016 +0200

    Gcc 6+ use C++14 as default, so there is no need to enforce C++11
    
    Also, use gnu++11 unconditionnally with gcc as we used to do before 
67385e69.
---
 config/lyxinclude.m4 |   10 +++++-----
 1 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/config/lyxinclude.m4 b/config/lyxinclude.m4
index 629247b..59a3753 100644
--- a/config/lyxinclude.m4
+++ b/config/lyxinclude.m4
@@ -373,7 +373,9 @@ if test x$GXX = xyes; then
       4.6*)
         dnl Note that this will define __GXX_EXPERIMENTAL_CXX0X__.
         dnl The source code relies on that.
-        cxx11_flags="-std=c++0x";;
+        cxx11_flags="-std=gnu++0x";;
+      4.7*|4.8*|4.9*|5.*)
+        cxx11_flags="-std=gnu++11";;
       clang-3.0*|clang-3.1*|clang-3.2*|clang-3.3*)
         dnl presumably all clang versions support c++11.
        dnl boost contains pragmas that are annoying on older clang versions
@@ -383,10 +385,8 @@ if test x$GXX = xyes; then
         dnl which  is very annoying with Qt4.x right now.
         cxx11_flags="-std=c++11"
         AM_CXXFLAGS="$AM_CXXFLAGS -Wno-deprecated-register";;
-      *)
-        AS_CASE([$host], [*cygwin*],
-                [cxx11_flags="-std=gnu++11"],
-                [cxx11_flags="-std=c++11"]);;
+      *) # gcc 6 and newer use C++14 as default
+        ;;
     esac
     # cxx11_flags is useful when running preprocessor alone
     # (see detection of regex).