Begin transition to C++11 by warning if using a pre-C++11 GCC compiler

configure.ac

@@ -119,10 +119,20 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([
 #endif
 ])],[],[AC_DEFINE([socklen_t],[int],[Define to `int` if you don't have socklen_t])])
 
+dnl C++0x is now considered deprecated.
+dnl This warning will stay here for a long enough period of time before making C++11 mandatory.
+
 dnl AC_CHECK_CXXFLAGS will prepend gnu++11 to CXXFLAGS if supported, for us.
 dnl our code adds -std=gnu++0x if not supported where the function would not do anything.
 AC_MSG_CHECKING([checking for C++11 support])
-AC_CHECK_CXXFLAGS([ -std=gnu++11 ], [  ], [ CXXFLAGS="$CXXFLAGS -std=gnu++0x" ])
+AC_CHECK_CXXFLAGS([ -std=gnu++11 ], [  ], [
+    CXXFLAGS="$CXXFLAGS -std=gnu++0x";
+    warn_cpp0x=1
+])
+
+if test x$warn_cpp0x = x1; then
+    AC_MSG_WARN([ C++0x is considered deprecated. Please use a C++11 compiler. C++0x support will be removed at a future date. ])
+fi
 
 dnl TEST: Environ can be included
 AC_MSG_CHECKING(if environ can be included)