Allow setting the GNATCOLL_ICONV_OPT environment variable when installing RecordFlux

[dalybrown]

The following line was causing me issues when installing RecordFlux via pip:

https://github.com/AdaCore/RecordFlux/blob/ca7f2ee9aa7bdf8552cdc32892a9da3a16b340e3/setup.py#L30C21-L30C21

I had to build gnatcoll_iconv.gpr myself separately (the static-pic library) in order to set the GNATCOLL_ICONV_OPT environment variable such that the symbols are found on my system.

Perhaps instead of setting the value directly, it could be a default value so that users can set it according to their system if required so that when installing RecordFlux the version of gnatcoll_iconv that comes bundled with it is built properly.

[treiher]

Thank you for the report! This issue has been fixed and will be part of the next release (0.15.0).