configure.ac: Do s/-pthreads -lpthreads/-pthreads/ for protobuf

[cgull]

protobuf uses an obsolete automake pthreads detection macro,
which results in "pkgconfig --libs protobuf" returning
"-lprotobuf -pthread -lpthread" on Linux. Remove
the unnecessary and dangerous -lpthread in that case.

Fixes #727, mosh-server crash in libutempter on Debian Sid.

[keithw]

This looks like the right fix, at least for now (although if the bug is in glibc, I hope we can get them to fix it!).

What did you mean by "an obsolete automake pthreads detection macro"? It's protobuf that is shipping that .pc file and specifying Libs: -L${libdir} -lprotobuf -pthread -lpthread. Or are you saying .pc files themselves are obsolete?

[keithw]

(And thanks for doing all that spelunking! Worthy of a KON and BAL column.)

[cgull]

The protobuf configure.ac uses an ancient ax_pthread.m4 macro, which Autoconf obsoleted and renamed to acx_pthread.m4 back in 2009. Google's made a few updates to their file, GNU Autoconf...a lot more. The variables that macro sets are used to substitute protobuf.pc.

Swapping in acx_pthread.m4 makes protobuf.pc more correct.

I will loudly kvetch that autoconf is a pile of accreted...stuff that seems to hinder portability as much as it helps; having the .m4 files be embedded by autoconf users rather than distributed with autoconf doesn't exactly help maintainability. It won't do any good, the OSS world is kind of stuck with it now.

[ghostbar]

Would love to see this merged; traveling without a working mosh sucks big time.

[cgull]

I'm going to wait till Monday for comment here and other places.

Since Sid is an unstable code base, I think this workaround is fine for now:
mosh --server="LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libutempter.so.0 /usr/bin/mosh-server" localhost

[ghostbar]

@cgull thank you! I had forgotten about the --server flag :)