+# Require C11 or newer. Autoconf-2.70 provides ac_cv_prog_cc_c11 when using
+# AC_PROG_CC, but we're still using Autoconf-2.69, and the newest it understands
+# is C99. So handle it ourselves.
+:
+C_DIALECT=
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether C11 is supported by default" >&5
+$as_echo_n "checking whether C11 is supported by default... " >&6; }
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h. */
+
+#if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112L
+# error "C11 support not found"
+#endif
+
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+ { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+else
+
+ { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+ { $as_echo "$as_me:${as_lineno-$LINENO}: checking for -std=c11 support" >&5
+$as_echo_n "checking for -std=c11 support... " >&6; }
+ ac_save_CC="$CC"
+ CC="$CC -std=c11"
+ cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h. */
+
+#if !defined __STDC_VERSION__ || __STDC_VERSION__ < 201112L
+# error "C11 support not found"
+#endif
+
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+
+ { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+ CC="$ac_save_CC"
+ C_DIALECT="-std=c11"
+
+else
+ as_fn_error $? "C11 is required" "$LINENO" 5
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+
+