Use "adoc" extension for asciidoc, it's the GitLab convention.

Makefile

@@ -9,7 +9,7 @@ ifeq ($(UNAME_S),Linux)
 endif
 
 OBJS=main.o init.o actions1.o actions2.o score.o misc.o database.o
-SOURCES=$(OBJS:.o=.c) COPYING NEWS README TODO adventure.text advent.text control advent.h funcs.h
+SOURCES=$(OBJS:.o=.c) COPYING NEWS README TODO adventure.text advent.adoc control advent.h funcs.h
 
 .c.o:
 	$(CC) $(CCFLAGS) $(DBX) -c $<
@@ -48,9 +48,9 @@ check: advent
 	cd tests; $(MAKE) --quiet
 
 # Requires asciidoc and xsltproc/docbook stylesheets.
-.asc.6: advent.txt
+.asc.6: advent.adoc
 	a2x --doctype manpage --format manpage $<
-.asc.html: advent.txt
+.asc.html: advent.adoc
 	a2x --doctype manpage --format xhtml -D . $<
 	rm -f docbook-xsl.css
 

README.adoc

@@ -5,7 +5,7 @@ This code is a forward-port of the Crowther/Woods Adventure 2.5 from
 development written by the original authors.  The authors have given
 permission and encouragement for this release.
 
-The file history.txt contains a more detailed history of this game
+The file history.adoc contains a more detailed history of this game
 and its ancestors.
 
 This project is called "Open Adventure" because it's not at all clear