Move the iconv test to the bottom of configure.

This fixes a possible mis-detection of iconv on OS X.
OS X with macports often has two version of libiconv.2.dylib
installed, one with symbols like "_iconv_open" and one with
"_libiconv_open", so test for iconv with all flags to make
sure the detection uses the same library as the actual
compilation / linking.

Tested-by: Paul Sturbaum