is allow_nonpic_in_shlib still useful?

In the plperl and plpython makefiles we look for a shared library of
libperl or libpython, and if it's not found, we check for
allow_nonpic_in_shlib, and if that's yes, then we proceed anyway.
Apparently, and IIRC, this was set up in a time when those shared
libraries were not available through standard builds, but I think that
hasn't been the case for quite a while.

The only platforms where we set allow_nonpick_in_shlib is linux and
freebsd/i386 (presumably an obsolescent combination).  Are there any
Linux builds that don't supply the required shared libraries?

I suspend this hack isn't useful anymore and ought to be removed.