Install libpq DLL with $(INSTALL_SHLIB).

Programs need execute permission on a DLL file to load it.  MSYS
"install" ignores the mode argument, and our Cygwin build statically
links libpq into programs.  That explains the lack of buildfarm trouble.
Back-patch to 9.0 (all supported versions).

src/interfaces/libpq/Makefile

@@ -126,7 +126,7 @@ install: all installdirs install-lib
 	$(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)'
+	$(INSTALL_SHLIB) $(shlib) '$(DESTDIR)$(bindir)/$(shlib)'
 endif
 
 installcheck: