Copy the libpq DLL to the bin directory on Mingw and Cygwin.

This has long been done by the MSVC build system, and has caused
confusion in the past when programs like psql have failed to start
because they can't find the DLL. If it's in the same directory as it now
will be they will find it.

Backpatch to all live branches.

src/interfaces/libpq/Makefile

@@ -121,12 +121,18 @@ install: all installdirs install-lib
 	$(INSTALL_DATA) $(srcdir)/libpq-int.h '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/pqexpbuffer.h '$(DESTDIR)$(includedir_internal)'
 	$(INSTALL_DATA) $(srcdir)/pg_service.conf.sample '$(DESTDIR)$(datadir)/pg_service.conf.sample'
+ifneq (,$findstring($(PORTNAME), win32 cygwin))
+	$(INSTALL_DATA) $(shlib) '$(DESTDIR)$(bindir)/$(shlib)'
+endif
 
 installcheck:
 	$(MAKE) -C test $@
 
 installdirs: installdirs-lib
 	$(MKDIR_P) '$(DESTDIR)$(includedir)' '$(DESTDIR)$(includedir_internal)' '$(DESTDIR)$(datadir)'
+ifneq (,$findstring($(PORTNAME), win32 cygwin))
+	$(MKDIR_P) '$(DESTDIR)$(bindir)'
+endif
 
 uninstall: uninstall-lib
 	rm -f '$(DESTDIR)$(includedir)/libpq-fe.h'
@@ -134,6 +140,9 @@ uninstall: uninstall-lib
 	rm -f '$(DESTDIR)$(includedir_internal)/libpq-int.h'
 	rm -f '$(DESTDIR)$(includedir_internal)/pqexpbuffer.h'
 	rm -f '$(DESTDIR)$(datadir)/pg_service.conf.sample'
+ifneq (,$findstring($(PORTNAME), win32 cygwin))
+	rm -f '$(DESTDIR)$(bindir)/$(shlib)'
+endif
 
 clean distclean: clean-lib
 	$(MAKE) -C test $@