s3 aclocal.m4: Fix iconv checks, clean up m4 code

The check for iconv requiring giconv.h and libgiconv as well as
the check for iconv requiring biconv.h and libbiconv were using the wrong
variable to check for previous successful test results. This caused the checks
to always fall back to libbiconv on systems where that library was available.

In the course of fixing this, I had to clean up the indentation in that piece of
code, and I also rewrote/added some comments.

Many thanks to Tsurutani Naoki <turutani@scphys.kyoto-u.ac.jp> for the initial
patch and diagnosis.