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.