I see the instructions are posted there.
Followed those instructions, stuck on the first make. See error below, looks like I need mingw-w64, but would appreciate step by step if anyone has.
user@ubuntu:~/Downloads/libiconv-1.15$ make
builddir="`pwd`"; cd libcharset && make all && make install-lib libdir="$builddir/lib" includedir="$builddir/lib"
make[1]: Entering directory '/home/dk/Downloads/libiconv-1.15/libcharset'
if [ ! -d include ] ; then mkdir include ; fi
cp ./include/libcharset.h.in include/libcharset.h
cd lib && make all
make[2]: Entering directory '/home/dk/Downloads/libiconv-1.15/libcharset/lib'
/bin/bash ../libtool --mode=compile x86_64-w64-mingw32-gcc -I. -I. -I.. -I./.. -I../include -g -O2 -fvisibility=hidden -DLIBDIR=\"/usr/x86_64-w64-mingw32/lib\" -DBUILDING_LIBCHARSET -DBUILDING_DLL -DENABLE_RELOCATABLE=1 -DIN_LIBRARY -DINSTALLDIR=\"/usr/x86_64-w64-mingw32/lib\" -DNO_XMALLOC -Dset_relocation_prefix=libcharset_set_relocation_prefix -Drelocate=libcharset_relocate -DHAVE_CONFIG_H -c ./localcharset.c
libtool: compile: x86_64-w64-mingw32-gcc -I. -I. -I.. -I./.. -I../include -g -O2 -fvisibility=hidden -DLIBDIR=\"/usr/x86_64-w64-mingw32/lib\" -DBUILDING_LIBCHARSET -DBUILDING_DLL -DENABLE_RELOCATABLE=1 -DIN_LIBRARY -DINSTALLDIR=\"/usr/x86_64-w64-mingw32/lib\" -DNO_XMALLOC -Dset_relocation_prefix=libcharset_set_relocation_prefix -Drelocate=libcharset_relocate -DHAVE_CONFIG_H -c ./localcharset.c -DDLL_EXPORT -DPIC -o .libs/localcharset.o
In file included from ./localcharset.c:27:
/usr/share/mingw-w64/include/stdio.h:9:10: fatal error: corecrt_stdio_config.h: No such file or directory
9 | #include <corecrt_stdio_config.h>
| ^~~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [Makefile:62: localcharset.lo] Error 1
make[2]: Leaving directory '/home/?/Downloads/libiconv-1.15/libcharset/lib'
make[1]: *** [Makefile:34: all] Error 2
make[1]: Leaving directory '/home/dk/Downloads/libiconv-1.15/libcharset'
make: *** [Makefile:42: lib/localcharset.h] Error 2