ESP8266 Community Forum

Unfortunately, on Windows, it is even more difficult.
The lwip_gcc compile is done using a Makefile located in ~/.arduino15/packages/esp8266/hardware/esp8266/2.3.0/tools/sdk/lwip/src/Makefile.
On Windows, you will need to have cygwin or mwing, so some more pain to install those.
On Linux, it would be pretty easier.

But even myself, I've never tried to compile lwip_gcc yet.

EDIT : I just gave it a try : pretty straight forward on Linux, "make" produce liblwip_src.a, then simply copy it to replace tools/lib/liblwip_gcc.a
Maybe if you have a Linux VM under VirtualBox, you can simply copy the tools folder, compile, and copy the resulting lib into your Windows.

Thanks, I do have a Linux VM installed on the Windows machine so I'll give it a try and let you know how it works.

Well, it appears the Makefile isn't correct for what I have. It had no idea where the xtensa binaries were located, couldn't find any of the lwip headers, and just kept dying.

I'm going to try the git copy and see if that works.

That didn't work either. I don't end up with the xtensa tools to use for compiling. I must be missing something entirely. I cloned the Git repository but it doesn't appear to give me a way to build the xtensa tools so I can build the rest of the code.