After >>184 there seem to be no more compiler errors in Gens 2.14 make. However:
/usr/bin/ld: i386 architecture of input file `gens_core/gfx/blit.o' is incompatible with i386:x86-64 output
At this point a 32-bit version should compile, but in that case the 363x linux kega fusion is the better option.
https://archive.org/download/kega-fusion
$ md5sum Fusion363x.tar.gz
9a6f93c2f3bd48ef09a9170e7f0512f4 Fusion363x.tar.gz
$ sha1sum Fusion363x.tar.gz
3821e866d6d9d6de11227a3e0c272613896c3e7e Fusion363x.tar.gz
The days of 32-bit support in Ubuntu are numbered anyway.
https://canonical.com/blog/statement-on-32-bit-i386-packages-for-ubuntu-19-10-and-20-04-lts
Statement on 32-bit i386 packages for Ubuntu 19.10 and 20.04 LTS
The 32-bit assembly in Gens:
$ wc $(find . -name '*.asm' | tr '\n' ' ')
141312 321960 2190466 ./src/starscream/sub68k/sub68k.asm
141679 322704 2198556 ./src/starscream/main68k/main68k.asm
1260 5145 30548 ./src/gens/gens_core/misc/misc.asm
1437 3434 26267 ./src/gens/gens_core/io/io.asm
493 997 6666 ./src/gens/gens_core/sound/pwm.asm
176 487 2921 ./src/gens/gens_core/vdp/vdp_32x.asm
1691 4670 29871 ./src/gens/gens_core/vdp/vdp_io.asm
2255 8223 48592 ./src/gens/gens_core/vdp/vdp_rend.asm
501 1151 7960 ./src/gens/gens_core/mem/mem_z80.asm
1944 4912 37248 ./src/gens/gens_core/mem/mem_m68k.asm
3628 7972 56537 ./src/gens/gens_core/mem/mem_s68k.asm
2196 4431 33772 ./src/gens/gens_core/mem/mem_sh2.asm
6237 18778 124315 ./src/gens/gens_core/cpu/z80/z80.asm
6131 16657 85597 ./src/gens/gens_core/cpu/sh2/sh2a.asm
1957 3559 41044 ./src/gens/gens_core/gfx/hq2x16.asm
976 2636 16912 ./src/gens/gens_core/gfx/gfx_cd.asm
2053 7906 47877 ./src/gens/gens_core/gfx/blit.asm
315926 735622 4985149 total
Rewriting the rest of this in 64-bit assembly other than the 68k emulation might be reasonable if the Programmer's Reference Manuals were available for all the chips. But rewriting the 68k emulation doesn't look like a reasonable time investment.