> Why is pcre linking to this?  If pcre is actually starting threads,
> there may be unexpected side-effects.

I don't know :(
But I know that dynamic version of libpcre links to pthread as well:

> $ ldd /lib64/libpcre.so.1
>        linux-vdso.so.1 (0x00007ffd63be6000)
>        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f7e8015e000)
>        libc.so.6 => /lib64/libc.so.6 (0x00007f7e7fd9a000)
>        /lib64/ld-linux-x86-64.so.2 (0x00007f7e805f0000)

// And even libpcre2 links to it.

And I don't see any kind of `--enable-threads` or `--enable-pthread` in their 
ebuilds (package manager build scripts), so, I guess, they links to pthread on 
their own, and not on maintainers decision.