This program should be compiled using the MinGW C compiler for Windows.

First compile the dll file:

gcc.exe -c essfunc.c
gcc.exe -shared -o essfunc.dll -Wl,--out-implib=libessfunc.a -Wl,--image-base=0x62500000 essfunc.o

Now compile the main program:

gcc.exe vulnserver.c -o vulnserver.exe -lws2_32 ./libessfunc.a


You can even compile this on a Linux machine using Wine and MinGW.  To do so, install Wine and the Windows MinGw and preface the commands above with:

wine ~/.wine/drive_c/MinGW/bin/