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.