Makefile: Enable debugging of parser.

This compiles the debugging code for teh parser. It's not active
unless the yydebug variable is set to a non-zero value.

Makefile

@@ -3,7 +3,7 @@ override CFLAGS += -Wall -Wextra -Wwrite-strings -Wswitch-enum -Wno-unused
 glcpp: glcpp.o glcpp-lex.o glcpp-parse.o hash_table.o
 
 %.c %.h: %.y
-	bison --defines=$*.h --output=$*.c $^
+	bison --debug --defines=$*.h --output=$*.c $^
 
 %.c: %.l
 	flex --outfile=$@ $<