Clean up MinGW def file generation

I was mystified by this comment in Makefile.shlib:

# We need several not-quite-identical variants of .DEF files to build
# DLLs for Windows.  These are made from the single source file
# exports.txt.  Since we can't assume that Windows boxes will have
# sed, the .DEF files are always built and included in distribution
# tarballs.

ifneq (,$(SHLIB_EXPORTS))
distprep: lib$(NAME)dll.def lib$(NAME)ddll.def
...

This doesn't make much sense (anymore?) since MinGW surely has sed and
MSVC doesn't use this (and has Perl).  I think this is a leftover from
various ancient client-only ad-hoc Windows build provisions (those
win32.mak files we used to have around).  Also, the ddll.def (debug
build) isn't used by anything anymore AFAICT.

I think we can clean this up and just have the regular ddl.def built
normally at build time if required.

Does anyone know more about this?

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
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