Found by pure coincidence, that gnuchessx builds the book even with
input from /dev/null. (This broke the port in dpb)

Tested on amd64 by building with make < /dev/null and with dpb
games/gnuchess.

Index: Makefile
===================================================================
RCS file: /cvs/ports/games/gnuchess/Makefile,v
diff -u -p -r1.56 Makefile
--- Makefile	8 Sep 2025 10:35:20 -0000	1.56
+++ Makefile	12 Mar 2026 13:40:15 -0000
@@ -1,9 +1,7 @@
-BROKEN=		post-build fails in dpb (or if stdin is redirected from /dev/null)
-
 COMMENT=	chess program
 
 DISTNAME=	gnuchess-6.3.0
-REVISION=	0
+REVISION=	1
 DISTFILES=	${DISTNAME}.tar.gz book_1.02.pgn.gz
 
 CATEGORIES=	games
@@ -27,7 +25,7 @@ CONFIGURE_STYLE= gnu
 
 post-build:
 	cd ${WRKSRC}/src && ln -sf ../../book_1.02.pgn
-	cd ${WRKSRC}/src && ./gnuchess --addbook book_1.02.pgn
+	cd ${WRKSRC}/src && ./gnuchessx --addbook book_1.02.pgn
 
 post-install:
 	${INSTALL_DATA_DIR} \