Re: Lower priority of the configure option --with-libraries causes wrong lib resolution

I wrote:
> I think we need something like the attached to ensure
> that -L switches coming from libraries' configure helpers
> don't override user-specified directories.

I looked this over again, and was about ready to commit it when
I realized that we have essentially the same problem for -I
switches. If there's a --with-includes switch, then the -I
switches from that should override anything supplied by external
configuration helpers, but we didn't reliably get that right.
(Some of those bugs are new in v18, but others are older.)

However, -I switches pointing to in-tree directories should
override all of those. We fixed one instance of that problem
a few months ago in cb36f8ec2, but I was dismayed to find that
there are more. This could result in build failures thanks to
pulling in the wrong version of some Postgres header.

Hence, 0001 attached is the same as before (but now with
a commit message), and then 0002 tackles the problems with -I
switches.

regards, tom lane