This commit enables use of the AX_CXX_COMPILE_STDCXX macro from the
autoconf-archive (added in the previous commit), which automatically
adds needed flags to the CXX variable to enable C++11 support. This only
works and is only required on compilers that are both new enough to have
C++11 support and old enough to not enable it by default.

We call it with "noext" as the second argument (so that we get the "std"
C++11 variant on GCC) and "mandatory" as the third argument, since we
now rely on C++11 features as of 83c59e70.

Note that this macro is only called when we are building the compiler
(i.e., without --disable-protoc) so that the library can still be built
on ancient C compilers.