configure errors because of syntax errors in the use of test shell command

[ackalker]

Please use this template for reporting suspected bugs or requests for help.

Issue description

On my system, where /bin/sh -> /bin/dash, runnung ./configure produces errors which I tracked down to configure.ac and acinclude.m4 using the test shell command with a == operator, syntax which is not valid in a POSIX-like shell like dash. See man 1p test for details.

Environment

• libzmq version (commit hash if unreleased): cef8065
• OS: Void Linux x86_64

Minimal test code / Steps to reproduce the issue

1. Run ./autogen.sh
2. Run ./configure

What's the actual result? (include assertion message & call stack if applicable)

[...output from step 2...]
./configure: 21529: test: x: unexpected operator
./configure: 21534: test: unexpected operator

Full output of running the repro steps on my system:
step 1: output
step 2: output.

What's the expected result?

Running configure should produce no such errors.

[bluca]

Could you please send a PR to fix it?

[ackalker]

Will do, I'm already on it.