A little update, I copied over libSDL_net.dll.a to the /mingw/lib and ran configure again and it picked up SDL_net, but then dosbox.exe needs sdl_net.dll.

you probably need some more makefile magic after you've run configure. At least for OS X (which is very restrictive about static linking) I need to edit the makefile and in the LIBS= section need to replace for example -lSDL_net with the path to the .a file (in my case /opt/exult.i386/lib/libSDL_net.a).
AND in the beginning of that (LIBS=) I need to replace the output of "sdl-config --libs" with what "sdl-config --static-libs" gives.

So instead of (very OS X centric)

1 LIBS = -lSDL_sound  -L/opt/exult.i386/lib -lSDLmain -lSDL -Wl,-framework,Cocoa -lpng -lz -lSDL_net -framework OpenGL -framework CoreMIDI -framework AudioUnit -framework AudioToolbox

I have

1LIBS = -L/opt/exult.i386/lib /opt/exult.i386/lib/libSDLmain.a /opt/exult.i386/lib/libSDL.a -Wl,-framework,OpenGL -Wl,-framework,Cocoa -Wl,-framework,ApplicationServices -Wl,-framework,Carbon -Wl,-framework,AudioToolbox -Wl,-framework,AudioUnit -Wl,-framework,IOKit -framework CoreMIDI /opt/exult.i386/lib/libpng.a /opt/exult.i386/lib/libz.a /opt/exult.i386/lib/libSDL_net.a /opt/exult.i386/lib/libSDL_sound.a /opt/exult.i386/lib/libogg.a /opt/exult.i386/lib/libvorbis.a /opt/exult.i386/lib/libvorbisfile.a /opt/exult.i386/lib/libvorbisenc.a

Again, this is all for OS X, you might want to look up what to do for mingw.

Just wanted to say a quick thanks, I saw there was a Makefile in the src folder which I edited it like you suggested, but this caused the compile to fail, I'll just keep trying and hopefully get somewhere.