Make iconv(3) tests portable..
We yet have had the problem that the several different iconv(3)
implementations out there use different replacement characters
and modes (byte-wise, character-wise), which we yet tried to
address by check iconv(1) output in t_behave_iconv_mainbody()
of cc-test.sh.
This however does not reflect what iconv(3) produces indeed.
So extend our make-config.sh test for iconv(3) and test right
away which kind of replacement that produces, and pass the
result on to cc-test.sh, which includes mk-config.ev.