Replace ancient m4/acx_pthread.m4 with m4/ax_pthread.m4

[cgull]

This is a notional PR illustrating an issue we're having with Mosh on debian-sid; the root cause seems to lie with protobuf (though there are at least two bugs/changes elsewhere in the build chain that also contribute).

Protobuf uses the very old, obsoleted-in-2009 ACX_PTHREAD macro to detect pthread libraries and set variables appropriately. On Debian Sid (and probably nearly any modern Linux distro), pkg-config --libs protobuf returns -lprotobuf -pthread -lpthread. Usage of -lpthread has been deprecated for a long, long time, but protobuf has been getting away with it until now. Something has recently changed in sid's build toolchain or binutils or glibc so that the oddly-placed -lpthread combined with Mosh's aggressive full-RELRO hardening causes a failure in symbol resolution elsewhere in Mosh, causing mosh-server to segfault on an unresolved PLT entry for fork() at runtime. By adopting the newer AX_PTHREAD macro from autoconf, we get a more-correct -lprotobuf -pthread, and mosh is happier.

See mobile-shell/mosh#727 and https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=817929 for much more discussion on the issue.

I have no idea what the best path for protobuf's maintainers to fix this is, but this PR should illustrate the problem nicely.

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[cgull]

I signed the CLA.

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[cgull]

Any interest in this issue? It's been dormant for 1.5 years.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[grpc-kokoro]

Thanks for your pull request. The automated tests will run as soon as one of the admins verifies this change is ok for us to run on our infrastructure.

[liujisi]

OK to test