Failing to compile a 15+ year old MUD codebase written in C

I used this codebase to run a mud in the early 2000s, and through that, to teach myself some very elementary coding.

I just got a copy of the codebase from the administrator/coder who ran the server before me, and I'm just about certain that it would compile as-is back then, but now it won't.

My only guess is that it has something to do with updates to GCC in the last 15-20 years, but I'm admittedly a novice and pretty out of my depth here.

This is the error I get after about a hundred pages of warnings when I try to compile it using make:

[18:24:54] finger.c ...
In file included from finger.c:10:0:
../src/include/merc.h:5149:13: warning: inline function ‘check_toughness’ declared but never defined
 inline int  check_toughness args( ( CD *ch, CD *victim, int dam));
             ^~~~~~~~~~~~~~~
[18:24:54] Linking Executable ...
obj/fight.o: In function `group_gain':
tbw/reloaded/src/fight.c:5514: undefined reference to `xp_compute'
tbw/reloaded/src/fight.c:5516: undefined reference to `xp_compute'
tbw/reloaded/src/fight.c:5518: undefined reference to `xp_compute'
obj/fight.o: In function `one_hit':
tbw/reloaded/src/fight.c:2457: undefined reference to `check_toughness'
obj/mage.o: In function `do_chant':
tbw/reloaded/src/mage.c:345: undefined reference to `check_toughness'
tbw/reloaded/src/mage.c:361: undefined reference to `check_toughness'
tbw/reloaded/src/mage.c:377: undefined reference to `check_toughness'
tbw/reloaded/src/mage.c:393: undefined reference to `check_toughness'
obj/mage.o:tbw/reloaded/src/mage.c:409: more undefined references to `check_toughness' follow
obj/update.o: In function `mobile_update':
tbw/reloaded/src/update.c:756: undefined reference to `werewolf_regen'
obj/powerl.o: In function `do_planarstorm':
tbw/reloaded/src/powerl.c:680: undefined reference to `check_toughness'
collect2: error: ld returned 1 exit status
Makefile:27: recipe for target '../src/reloaded' failed
make: *** [../src/reloaded] Error 1

The codebase and the referenced files are huge, so I figured it wouldn't make sense to attach them just yet, but if required I can definitely provide them.

For the record, those functions like "check_toughness" and "xp_compute" are definitely defined within those files after a few thousand LOC.

Would appreciate any suggestions, thank you!