Revisit our manual resolution to ./configure

[cmb69]

Upstream libiconv only offers autoconf build system support. This has been manually resolved in our port, where e.g. iconv.h and config.h have been committed to this repo. Besides that even updating the libiconv version is easily overlooked when updating from upstream (e.g. b6ed966 and 880a1fa), more serious issues could arise, especially for bigger upstream changes.

Additionally, due to missing Makefiles we have VS solutions for each VS version. That appears to be a maintenance nightmare, since changes to one solution are not reflected in other solutions, and manual porting is tedious, as are manual diffs. A single VS solution should do nowadays, but maybe we're even better off with a Makefile.

Another, more holistic approach might be to use a CMake based build system. Ideally, that could be intregated upstream, but even if not, it might be better than manually fiddling with the configuration. https://github.com/vovythevov/libiconv-cmake might be a good starting point.