looking at
https://launchpadlibrarian.net/876718955/buildlog_ubuntu-stonking-amd64.gcc-mingw-w64_29_BUILDING.txt.gz

gcc: fatal error: cannot execute ‘gnat1’: posix_spawnp: No such file or
directory
compilation terminated.
gnatmake: "/<<PKGBUILDDIR>>/src/gcc/ada/gen_il-main.adb" compilation error
make[4]: *** [/<<PKGBUILDDIR>>/src/gcc/ada/Make-generated.in:13:
ada/stamp-gen_il] Error 4
make[4]: *** Waiting for unfinished jobs....
cp -p /<<PKGBUILDDIR>>/src/gcc/ada/snames.ads-tmpl
/<<PKGBUILDDIR>>/src/gcc/ada/snames.adb-tmpl
/<<PKGBUILDDIR>>/src/gcc/ada/snames.h-tmpl
/<<PKGBUILDDIR>>/src/gcc/ada/xsnamest.adb
/<<PKGBUILDDIR>>/src/gcc/ada/xutil.ads
/<<PKGBUILDDIR>>/src/gcc/ada/xutil.adb ada/bldtools/snamest
echo timestamp > s-i386-bt
cd ada/bldtools/snamest && gnatmake xsnamest && ./xsnamest
gcc -c xsnamest.adb
gcc: fatal error: cannot execute ‘gnat1’: posix_spawnp: No such file or
directory
compilation terminated.
gnatmake: "xsnamest.adb" compilation error
make[4]: *** [/<<PKGBUILDDIR>>/src/gcc/ada/Make-generated.in:41:
ada/stamp-snames] Error 4
yes

with GCC still defaulting to 15.  You assume that the GCC version
matches the version that you are building with.

Also I find the name for the target_version macro a bit confusing,
because it means both, the version for the target, and the version for
the build (the build still succeeds building with 15).

patch attached.