Append (instead of assigning) to CFLAGS, and declare (by appending) and use
LDFLAGS, so that user- (or MacPorts-)specified flags aren't ignored.

Change _POSIX_SOURCE to _POSIX_C_SOURCE=2L to fix:
main.c:65:17: error: implicit declaration of function 'getopt' is invalid in C99 [-Werror,-Wimplicit-function-declaration]

Add tokens.h to dependencies of tokdump.o.

Remove duplicate declaration of dependencies of interpret.o.

Declare dependencies of main.o to fix (when building in parallel):
main.c:24:10: fatal error: 'tokens.h' file not found

Remove tokens.h from dependencies of monitor.o.

Preface the comment "# Hmmm...this is probably Bison-specific" with "@" to
prevent it from appearing in the terminal during the build.

Add cupl.h to dependencies of lextest.
--- Makefile.orig	2019-06-12 10:39:17.000000000 -0500
+++ Makefile	2023-01-12 20:54:04.000000000 -0600
@@ -5,11 +5,12 @@
 
 CDEBUG = -g	# use -O for production, -g for debugging
 YFLAGS = -vt	# use -l for production, -vt for debugging
-CFLAGS = $(CDEBUG) -D_POSIX_SOURCE -DPARSEDEBUG	-DYYDEBUG=1
+CFLAGS += $(CDEBUG) -D_POSIX_C_SOURCE=2L -DPARSEDEBUG -DYYDEBUG=1
+LDFLAGS += -lm
 
 MODULES = main.o grammar.o lexer.o interpret.o tokdump.o execute.o monitor.o
 cupl: $(MODULES)
-	$(CC) $(MODULES) -lm -o cupl
+	$(CC) $(MODULES) $(LDFLAGS) -o cupl
 
 # You can use either lex or flex
 #LEX = lex
@@ -30,18 +31,18 @@
 	mv y.tab.h tokens.h
 
 grammar.o: grammar.c cupl.h
-tokdump.o: tokdump.c toktab.h
+tokdump.o: tokdump.c tokens.h toktab.h
 lexer.o: lexer.c tokens.h cupl.h
 interpret.o: interpret.c tokens.h cupl.h
-interpret.o: interpret.c tokens.h cupl.h
+main.o: main.c tokens.h cupl.h
 execute.o: execute.c tokens.h cupl.h
-monitor.o: monitor.c tokens.h cupl.h
+monitor.o: monitor.c cupl.h
 
 toktab.h: tokens.h
-	# Hmmm...this is probably Bison-specific
+	@# Hmmm...this is probably Bison-specific
 	awk <tokens.h >toktab.h '/^# define	/ {print $$3 ", \"" $$3 "\","}'
 
-lextest: lexer.c tokens.h tokdump.o
+lextest: lexer.c tokens.h cupl.h tokdump.o
 	$(CC) $(CFLAGS) -DMAIN lexer.c tokdump.o -o lextest 
 
 cupl.1: cupl.xml