tests: Apple clang issues warning on C++11 features

$ cat /tmp/foo.cc
using foo = int;
foo f;
$ clang++ -Wc++11-extensions -c /tmp/foo.cc
/tmp/foo.cc:1:13: warning: alias declarations are a C++11 extension [-Wc++11-extensions]
using foo = int;
            ^
1 warning generated.
$ clang++ --version
Apple clang version 11.0.0 (clang-1100.0.33.17)
Target: x86_64-apple-darwin18.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

* tests/local.at (AT_COMPILE_CXX): Make sure -std=c++11 is passed when
running glr2.cc.  It may be overridden by another flag in CXXFLAGS
afterwards.

tests/headers.at

@@ -386,7 +386,11 @@ AT_PERL_CHECK([[-n -0777 -e '
 # Do this late, so that other checks have been performed.
 AT_SKIP_IF_CANNOT_LINK_C_AND_CXX
 
+# The header generated by glr2.cc requires C++11.  Push the options to
+# prepare AT_COMPILE_CXX.
+AT_BISON_OPTION_PUSHDEFS([%skeleton "glr2.cc"])
 AT_COMPILE_CXX([parser], [[x[1-9a-d].o -DCC_IS_CXX=$CC_IS_CXX main.cc]])
+AT_BISON_OPTION_POPDEFS
 AT_PARSER_CHECK([parser], [0], [[expout]])
 
 m4_popdef([AT_TEST])