If you want Port modern Unix apps, best is you use ixemul and GCC 4.use libnix is slow when a program do lots of memalloc (C++ do) and miss lots functions newer Unix Progs need (C99)
Here you can find in folder compilers the Version 4.3.2 for cygwin.It work on amidevcpp.
https://sourceforge.net/projects/amiga/files/newer versions i have too and can easy compile from GCC source.
how can compile a GCC is add in the readme.