Merge pull request #334 from protobuf-c/edmonds/ax_cxx_compile_stdcxx

Use AX_CXX_COMPILE_STDCXX macro to enable C++11 support in old compilers

.travis.yml

@@ -13,7 +13,6 @@ addons:
 
 env:
   global:
-    - CXX="g++ -std=c++11"
     - PROTOBUF_VERSION=3.6.1
     - PKG_CONFIG_PATH=$HOME/protobuf-$PROTOBUF_VERSION-bin/lib/pkgconfig
 

configure.ac

@@ -76,6 +76,8 @@ AC_ARG_ENABLE([protoc],
 if test "x$enable_protoc" != "xno"; then
   AC_LANG_PUSH([C++])
 
+  AX_CXX_COMPILE_STDCXX(11, noext, mandatory)
+
   PKG_CHECK_MODULES([protobuf], [protobuf >= 3.0.0],
     [proto3_supported=yes],
     [PKG_CHECK_MODULES([protobuf], [protobuf >= 2.6.0])]