The asc2uni file name sounds like it is part of Unitex, and if so notice that the newest version have combined asc2uni and uni2asc into one program called convert. So you might want to investigate if you want to switch to a newer version before investing too much energy into trying to compile something old. Also I see there are a windows installer binary available as well, maybe you even do not have to compile anything?
Anyhow, to compile the file with the gcc compiler run
gcc -o asc2uni asc2uni.c
The file asc2uni.1 sound like a manual file and does not need to be compiled, just installed. If you want to create a plain text version of it you can run
nroff -man asc2uni.1 | col -b > asc2uni.txt
For windows you can install cygwin where you can select to install gcc. There are also other ports of gcc (like mingw), but probably any C compiler will do. Nroff and col are probably available in cygwin, but I am not 100% sure.