WIP: pg_regress updates for vc++ bulid

Hello!

Per some previous discussion that I can't really recall if it was on or
off list, here is a WIP patch to make pg_regress run completely outside
of msys on win32.

The change needed is that the processing of files from input/ and
output/ into sql/ and expected/ is moved from the Makefile and into
pg_regress itself.

I have tested on Win32 non-MSVC and on Linux (non-VPATH), and it appears
to work there.

Still to be done is the "make install" part on MSVC build, currently
#ifdef:ed out. While working on that, I'd appreciate some comments on
the patch in general - if there are more things I need to fix before it
can be considered. Particularly I'm concerned around the VPATH stuff,
since I've never used that myself.

//Magnus