allow gcc build to succeed with -DWIN31SUPPORT

[ecm-pushbx]

I copied config.m to config.mak and enabled the -DWIN31SUPPORT switch like so:

kwin31$ tail config.mak

# XFAT=16
XFAT=32

# Give extra compiler DEFINE flags here
# such as -DDEBUG : extra DEBUG output
#         -DDOSEMU : printf output goes to dosemu log
# set ALLCFLAGS=-DDEBUG
ALLCFLAGS=-DWIN31SUPPORT
NASMFLAGS=-DWIN31SUPPORT

(Is this correct? Is there a better way to set this flag?)

Prior to this commit building with -DWIN31SUPPORT would fail with errors from NASM:

nasm -Dgcc -DWIN31SUPPORT -DWITHFAT32 -i../hdr/ -DXCPU=386 -felf -o kernel.obj kernel.asm
kernel.asm:610: error: ELF format does not support segment base references
kernel.asm:613: error: ELF format does not support segment base references
kernel.asm:614: error: ELF format does not support this use of WRT

I did a small validity check on the resulting structure and it seems to be correct when I call int 2Fh function 1605h, however I do not know whether this works and have not tested or studied this extensively.

[ecm-pushbx]

For others following along, I use the following commands to build the kernel:

PATH=~/proj/build-ia16/prefix/bin/:"$PATH"
git clone https://github.com/FDOS/kernel kwin31
cd kwin31
git submodule init
git submodule update --recursive
make all COMPILER=gcc XUPX=

The first command sets up the path to the gcc-ia16 toolchain, set up using https://github.com/tkchia/build-ia16/

[ecm-pushbx]

I noticed that with the config.mak file the default appears to be to build the 386-only kernel.