IIUC the _AX_CXX_COMPILE_STDCXX_11_testbody macro from
m4/am_cxx_compile_stdcxx_11.m4 tests for C++11 conformance via a number of
C++11 tricks. I think it would be simpler to just test the __cplusplus macro
against its C++11 value.
static_assert(__cplusplus >= 201103L, "C++11?");
I reckon GCC had an issue with this macro for a very long time, but IIUC it's
now resolved, and we can rely on it at least for values greater or equal to
that of C++11.
I also think this change would make it easier to support checking for newer
standards, like checking for the compliance of C++14. Its value for
__cplusplus is 201402L, so it would be as simple as changing the static assert
for that.