I have made some changes to master branch. I can get it to compile with kernel 4.15.18+.
If I get permission I will upload my branch.

Well, it worked once, and now the module isn't working.

Using modprobe will load the module, but /dev/ttyQxx doesn't populate, and the sstty program says no breakout board was found. If I try loading the module using insmod it gives me:
insmod: ERROR: could not insert module ./drv/eqnx.ko: Invalid module format

dmesg only elaborates by saying
[ 289.905792] eqnx: no symbol version for module_layout

Literally all I did between it working and not working was reboot?

Hello !

Can you help build this driver for CentOS6 last stock kernel ?

[root@stat-vas eqnx-master]# make
cd drv; make build
make[1]: Entering directory /root/Workspace/eqnx-master/drv' make -C /usr/src/linux-2.6.32-754.17.1.el6.i686 SUBDIRS=/root/Workspace/eqnx-master/drv modules make[2]: Entering directory /usr/src/kernels/2.6.32-754.17.1.el6.i686'
CC [M] /root/Workspace/eqnx-master/drv/sst.o
/root/Workspace/eqnx-master/drv/sst.c:113:32: error: linux/sched/signal.h: No such file or directory
/root/Workspace/eqnx-master/drv/sst.c:8590: error: conflicting types for ‘tty_prepare_flip_string_flags’
include/linux/tty_flip.h:8: note: previous declaration of ‘tty_prepare_flip_string_flags’ was here
/root/Workspace/eqnx-master/drv/sst.c: In function ‘tty_prepare_flip_string_flags’:
/root/Workspace/eqnx-master/drv/sst.c:8593: warning: passing argument 1 of ‘tty_buffer_request_room’ from incompatible pointer type
include/linux/tty_flip.h:4: note: expected ‘struct tty_struct *’ but argument is of type ‘struct tty_port *’
/root/Workspace/eqnx-master/drv/sst.c:8595: error: ‘struct tty_port’ has no member named ‘buf’
/root/Workspace/eqnx-master/drv/sst.c:8596: warning: implicit declaration of function ‘char_buf_ptr’
/root/Workspace/eqnx-master/drv/sst.c:8596: warning: assignment makes pointer from integer without a cast
/root/Workspace/eqnx-master/drv/sst.c:8597: warning: implicit declaration of function ‘flag_buf_ptr’
/root/Workspace/eqnx-master/drv/sst.c:8597: warning: assignment makes pointer from integer without a cast
/root/Workspace/eqnx-master/drv/sst.c: In function ‘megainput’:
/root/Workspace/eqnx-master/drv/sst.c:8700: warning: passing argument 1 of ‘tty_prepare_flip_string_flags’ from incompatible pointer type
/root/Workspace/eqnx-master/drv/sst.c:8590: note: expected ‘struct tty_port *’ but argument is of type ‘struct tty_struct *’
At top level:
cc1: warning: unrecognized command line option "-Wno-incompatible-pointer-types"
make[3]: *** [/root/Workspace/eqnx-master/drv/sst.o] Error 1
make[2]: *** [module/root/Workspace/eqnx-master/drv] Error 2
make[2]: Leaving directory /usr/src/kernels/2.6.32-754.17.1.el6.i686' make[1]: *** [modules] Error 2 make[1]: Leaving directory /root/Workspace/eqnx-master/drv'
make: *** [build] Error 2

Running make build fails since gcc can't find sst.o (which hasn't been built yet?):

/home/user/git/eqnx/drv/Makefile:178: ***

CC [M] /home/user/git/eqnx/drv/sst.o

gcc: error: : No such file or directory

scripts/Makefile.build:257: recipe for target '/home/user/git/eqnx/drv/sst.o' failed

make[3]: *** [/home/user/git/eqnx/drv/sst.o] Error 1

make[3]: Target '__build' not remade because of errors.

Makefile:1353: recipe for target '_module_/home/user/git/eqnx/drv' failed

make[2]: *** [_module_/home/user/git/eqnx/drv] Error 2

make[2]: Target 'modules' not remade because of errors.

make[2]: Leaving directory '/usr/src/linux'

/home/user/git/eqnx/drv/makefile_2.6:38: recipe for target 'modules' failed

make[1]: *** [modules] Error 2

make[1]: Target 'build' not remade because of errors.

make[1]: Leaving directory '/home/user/git/eqnx/drv'

makefile:54: recipe for target 'build' failed

make: *** [build] Error 2

Any idea what the heck is going on? Building on Debian Jessie with kernel 3.16.0-4-amd64.

Only made one modification to the Makefile. I added:
MCFLAGS = "${CFLAGS}"
And modified the original MCFLAGS line like so:
MCFLAGS += ...
So I could pass -Wno-error to keep the build from failing on warnings.