OK it gets a bit further, now it moans about zlib.h
AS wiiu/system/stubs_rpl.S
MAKE wiiu/wut/elf2rpl/elf2rpl.exe
make[1]: Entering directory '/c/git/retroarch/wiiu/wut/elf2rpl'
CC wiiu/system/dynamic.c
g++ -c -o main.o main.cpp -O3 -Wall -std=c++14 -DNDEBUG -I.
AS wiiu/system/stubs_elf.S
LD objs/wiiu-salamander/retroarch_wiiu_salamander.rpx.elf
LD objs/wiiu-salamander/retroarch_wiiu_salamander.elf
cp objs/wiiu-salamander/retroarch_wiiu_salamander.elf retroarch_wiiu_salamander.elf
main.cpp:10:10: fatal error: zlib.h: No such file or directory
10 | #include <zlib.h>
| ^~~~~~~~
compilation terminated.
make[1]: *** [Makefile:24: main.o] Error 1
make[1]: Leaving directory '/c/git/retroarch/wiiu/wut/elf2rpl'
make: *** [Makefile.wiiu:319: wiiu/wut/elf2rpl/elf2rpl.exe] Error 2
make: Leaving directory '/c/git/retroarch'
--------------------- MERGED ---------------------------
OK hold the phone... I switched from using Msys2.exe to MingW64.exe and now it compiles elf2rpl
I'm trying a core now... wish me luck