Warn if using pre-C++11 so find out who needs it.

configure.ac

@@ -65,6 +65,21 @@ AC_ARG_ENABLE([longtests],
 AC_SUBST(CFG_WITH_LONGTESTS)
 AC_MSG_RESULT($CFG_WITH_LONGTESTS)
 
+# CFG_WITH_PREC11
+AC_MSG_CHECKING(whether allow pre-C++11)
+AC_ARG_ENABLE([prec11],
+              [AS_HELP_STRING([--enable-prec11],
+                              [enable pre-C++11 compilers])],
+              [case "${enableval}" in
+                yes) CFG_WITH_PREC11=yes ;;
+                no)  CFG_WITH_PREC11=no ;;
+                *)   AC_MSG_ERROR([bad value ${enableval} for --enable-prec11]) ;;
+               esac],
+              [CFG_WITH_PREC11=no;]
+              )
+AC_SUBST(CFG_WITH_PREC11)
+AC_MSG_RESULT($CFG_WITH_PREC11)
+
 # Compiler flags
 CFLAGS=-I${includedir}
 CPPFLAGS=-I${includedir}
@@ -284,6 +299,22 @@ CFG_WITH_THREADED=$_my_result
 AC_SUBST(CFG_WITH_THREADED)
 AC_MSG_RESULT($CFG_WITH_THREADED)
 
+# Check compiler flag
+if test "$CFG_WITH_THREADED" = "no" ; then
+   if test "$CFG_WITH_PREC11" = "no" ; then
+      AC_MSG_NOTICE([[]])
+      AC_MSG_ERROR([[the $CXX compiler appears to not support C++11.
+
+Verilator plans to require a C++11 or newer compiler in a future release,
+unless sufficient people report problems. Therefore, if you do not have a
+C++11 compiler, please post a message to
+https://www.veripool.org/boards/3/topics/2580-Verilator-Requiring-C-11-compiler
+indicating your OS and when you think C++11 might be ok, and then rerun
+configure with the --enable-prec11 argument. Thanks.]])
+
+   fi
+fi
+
 # Checks for library functions.
 AC_CHECK_MEMBER([struct stat.st_mtim.tv_nsec],
     [AC_DEFINE([HAVE_STAT_NSEC],[1],[Defined if struct stat has st_mtim.tv_nsec])],