Fix assorted portability issues in commit 896fcdb23.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 13c98bdfc41eefb75da24fd2a886b0ef5314b7f6
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2020-03-25T23:37:30Z
Releases: 13.0
Fix assorted portability issues in commit 896fcdb23.

Some platforms require libssl to be linked explicitly in the new
SSL test module.  Borrow contrib/sslinfo's code for that.

Since src/test/modules/Makefile now has a variable SUBDIRS list,
it needs to follow the ALWAYS_SUBDIRS protocol for that (cf.
comments in Makefile.global.in).

Blindly try to fix MSVC build failures by adding PGDLLIMPORT.

Files