d796091b04f3b02d2280aaa761c2b94950199da8 build: Bump AX_PTHREAD macro to the latest version (Hennadii Stepanov)
Pull request description:
This PR silents autoconf >2.69 (this [one](https://formulae.brew.sh/formula/autoconf), for instance) warnings about the obsolete `$as_echo`:
```
% ./autogen.sh
...
configure.ac:847: warning: $as_echo is obsolete; use AS_ECHO(["message"]) instead
lib/m4sugar/m4sh.m4:692: _AS_IF_ELSE is expanded from...
lib/m4sugar/m4sh.m4:699: AS_IF is expanded from...
./lib/autoconf/general.m4:2249: AC_CACHE_VAL is expanded from...
./lib/autoconf/general.m4:2270: AC_CACHE_CHECK is expanded from...
build-aux/m4/ax_pthread.m4:89: AX_PTHREAD is expanded from...
configure.ac:847: the top level
...
```
No other behavior changes.
ACKs for top commit:
fanquake:
ACK d796091b04f3b02d2280aaa761c2b94950199da8 - matches upstream at serial 31.
Tree-SHA512: aa9b60698f453427221444a5a63420d833c4c5dd23f8b0c74e5bd4639daec9c6cff0907a5281c00103ccb030e394998cf05653be750d4a3bf0f37ca41ff6fbe1