Configure: fixed posix_spawn() detection with glic 2.30.
Configure: fixed posix_spawn() detection with glic 2.30.

In particular, it was previously broken on Ubuntu 19.10 and Fedora 31.
See for details: https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=2ab5741

nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit,nginx/unit