[RFA] Add --with-libz-prefix option in config/zlib.m4

[Joel Brobecker]

> > This patch enhances config/zlib.m4 to introduce an extra option
> > --with-libz-prefix which allows us to provide the location of
> > the zlib library we want to use during the build.
> 
> I prefer the gcc way to provide external library:
> 
> --with-zlib -> system zlib used
> --with-zlib=pathname -> zlib from pathname is used

Unfortunately, we took a different route with GDB (with-...-prefix).
For instance:

  --with-libiconv-prefix
  --with-liblzma-prefix
  --with-libbabeltrace-prefix

We already have a really nice macro that takes care of a lot of
configury for us with respect to lib depenencies, which I see
in gcc's root configure.ac that gmp/mpfr/mpc options is done
via specific code.

What I'm getting at is that, to achieve your prefered option
(which I agree is more elegant), it would take a significant
effort.

-- 
Joel