(Since you asked for testing of various platforms and compilers...)
Compiling with Open Watcom 1.9 produces some warnings. I have not
investigated any of the warnings. I didn't use the makefile, I just
moved luac.c out of the way and compiled *.c. For comparison,
compiling Lua 5.2.3 does not report any warnings.
$ owcc -Wall -Wextra -o lua *.c
lcode.c(766): Warning! W200: 'a' has been referenced but never
assigned a value
lcode.c(766): Warning! W200: 'b' has been referenced but never
assigned a value
ldo.c(709): Warning! W124: Comparison result always 0
lgc.c(762): Warning! W124: Comparison result always 0
llex.c(63): Warning! W124: Comparison result always 0
llex.c(176): Warning! W124: Comparison result always 0
lstate.c(246): Warning! W124: Comparison result always 0
lzio.c(73): Warning! W124: Comparison result always 0