Configure included iconv with autotools

Configure included iconv with autotools

The included iconv should not be used on Linux or OS X, but (depending on
local configuration) it might be needed for crosscompiling a mingw target
from Linux. Now the user can choose whether to use the included iconv or not.
cmake does already support that.

eilseq.m4 was taken from the original libiconv 1.14 package.