win32 patch: allows source to be compiled + "run" under MingW

For application to HEAD, pending community review (rand.c to go in src/port)

Briefly,
* configure + Makefile changes
* shared memory attaching in EXEC_BACKEND case (+ minor fix for apparent
cygwin bug under cygwin/EXEC_BACKEND case only)
* PATH env var separator differences
* missing win32 rand functions added
* placeholder replacements for sync etc under port.h

To those who are really interested, and there are a few of you: the attached
patch + file will allow the source base to be compiled (and, for some
definition, "run") under MingW, with the following caveats (I wanted to
first properly fix all but the last of these, but y'all won't quit asking
for a patch :-):

* child death: SIGCHLD not yet sent, so as a minimum, you'll need to
put in some sort of delay after StartupDatabase, and handle setting
StartupPID to 0 etc (ie. the stuff the reaper() signal function is supposed
to do)

* dirmod.c: comment out the elog calls

* dfmgr.c: some hackage required to substitute_libpath_macro

* slru/xact.c: comment out the errno checking after the readdir
(fixed by next version of MingW)

Again, this is only if you *really* want to see postgres compile and start,
and is a nice leg-up for working on the other Win32 TODO list items. Just
don't expect too much else from it at this point...

Cheers,
Claudio

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