While trying to build some old stuff I ran into this weird issue where YYLESX is undefined:

gcc -DHAVE_CONFIG_H -I. -I. -I../../include -I../.. -I../.. -I../../include -g -O2 -Wall -c cs_grammar.c
cs_grammar.y: In function ‘cs_parse’:
cs_grammar.y:1074:58: error: ‘YYLEX’ undeclared (first use in this function)
1074 | yychar = YYLEX;
| ^~~~~
cs_grammar.y:1074:58: note: each undeclared identifier is reported only once for each function it appears in
make[3]: *** [Makefile:334: cs_grammar.o] Error 1

Notice it’s all UPPERCASE, but you can find plenty on the lowercase issue where its not being linked correctly. And yeah turns out YYFLEX should define out toe yyflex

# ifndef YYLEX
#  define YYLEX		yylex()
# endif

Add that in the top of whatever source is complaining and you’re golden.

Really.