Use modified AM_ICONV instead of AC_CHECK_FUNC for iconv

Modifications from /usr/share/aclocal/ version:
1. Don't use AC_LIB_RPATH
2. AC_SUBST([HAVE_ICONV])

So now charset module is buildable on openbsd with:
./configure CPPFLAGS=-I/usr/local/include LDFLAGS=-L/usr/local/lib

Fix #255