Fix for forced libiconv in non Visual Studio environments

[jtorresfabra]

Hi @SpaceIm

First of all thanks for your support making gdal available through conan. We are experiencing some licensing issues due to the fact that the conan recipe is not honoring the --with-libiconv option. I'm not sure what is the reason for disabling this option in non VisualStudio builds. This PR addresses the problem we have, but wondering why it was done in the first place.

Thanks a lot.

Commit message:
The problem with forcing libiconv (LGPL 2.1) is mainly a matter of licensing, making impossible to statically build gdal (MIT license) in enviroments where LGPL is not permitted. The proposed fix honors the option to disable libiconv.

[jtorresfabra]

By the way, not sure is ok to do the PR here or directly to conan-center, but as it seems you are the main maintainer I decided to do it here.

Let me know if this is not the correct place! Thanks.

[SpaceIm]

Hi, thanks for the PR. You're right, libiconv can be disabled in autoconf build
I can merge here, but the reference is conan-center-index. I'm using this repo to experiment few things before submitting modifications of gdal recipe to conan-center-index, and actually this repo lacks few recent improvements of CCI recipe.

You can submit your modifications in conan-center-index, or I can open a PR, as you wish.

And I would also replace:

        if self.options.get_safe("with_libiconv", True):
            self.requires("libiconv/1.16")

by

        if self.options.with_libiconv:
            self.requires("libiconv/1.16")

[jtorresfabra]

Hi @SpaceIm,

Thanks! ok I can do it, probably will tag you to review there too. Are you planning to open a PR for gdal 3.3.1 in the near term?

[SpaceIm]

Yes, but due to the amount of patches required for each version, it takes time. I've also seen that, since 3.3.0, several drivers were moved to https://github.com/OSGeo/gdal-extra-drivers, so it requires extra work.

[SpaceIm]

Suggested change

	if self.options.with_libiconv:
	args.append("--with-libiconv-prefix={}".format(tools.unix_path(self.deps_cpp_info["libiconv"].rootpath)))
	else:
	args.append("--without-libiconv-prefix")
	args.append("--with-libiconv-prefix={}".format(tools.unix_path(self.deps_cpp_info["libiconv"].rootpath) if self.options.with_libiconv else "no"))

can be simplified in one line

[jtorresfabra]

I think you actually need to be verbose in disabling it. Meaning explicitely call "without-libiconv-prefix".

[jtorresfabra]

https://github.com/OSGeo/gdal/blob/master/gdal/m4/iconv.m4#L35-L40

[SpaceIm]

--with-libiconv-prefix=no should be the same than --without-libiconv-prefix