GNU autoconf-2.70 released

Compilers have this nasty tendency to set the value of __cplusplus (or __STDC_VERSION__ for C) aspirationally, before they're done implementing the new version of the standard. (It makes sense on the development branch: you need the new value of the macro in place early so you can implement things that key off of it in the headers. The trouble is, then that gets into a production release before it's fully debugged.) Autoconf's test for C2011-conformant anonymous unions makes a good example of the problems we have to weed out.

"Why don't you have a separate AC_C_ANONYMOUS_UNIONS test for that?" I hear you ask. We do offer that kind of specific test, but it's intended for features that were first implemented in isolation and got rolled into a standard update later, like inline in C. If you are writing new code in C++11 or whatever, we'd like you to be able to rely on __cplusplus and not have to do a bunch of configure-time tests that are, in principle, redundant to the version of the standard you asked for.