Fix bug: when checking compatibility between GLOBAL and EXTERNAL variables, must only enforce GLOBAL CONSTANT => EXTERNAL CONSTANT.
Reverse CONSTANT implication is not enforced, and neither is RETAIN compatibility.

STD_TEST.st
STD_RESSOURCE.c
LOCATED_VARIABLES.h
POUS.c
test
STD_CONF.c
SFC_TEST.st
test.exe
plc.st
POUS.h
VARIABLES.csv