Add crosscompiling support for ./configure.in

[icegood]

For now during configuration of the module for other platform we will obtain:

checking whether listen supports negative backlog... 
configure: error: in /home/ice/external/openwrt_build/under_env/openwrt_22_series_for_dlink620/build_dir/target-mipsel_24kc_musl/proftpd-28e60b76':
configure: error: cannot run test program while cross compiling

the issue could be fixed if all of macroses AC_TRY_RUN will be feed with
AC_MSG_RESULT(cross-compiling);
and some random guesses under cross compilation

[Castaglia]

Can you explain your use cases/needs, for why/when you'd like such cross-compiling support?