need to unbreak Emacs build on AIX

[Paul Eggert <eggert@cs.ucla.edu>]

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Commit 2020-09-12T07:11:26Z!eliz@gnu.org 
(694acda5f24e356264c3f2a55a49812e02bcb49a) broke the build on AIX 7.2, 
as AIX uses libncurses for terminfo and its libncurses does not define 
UP, BC, or CP. Normally I'd just install the attached patch into the 
emacs-27 branch and then merge to master, but I thought I'd check on the 
mailing list since I don't offhand know how close we are to a 27.2 release.


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From 2504eef721805401397822accbcadc14a0cc1d0d Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Fri, 1 Jan 2021 15:15:54 -0800
Subject: [PATCH] Fix broken build on AIX 7.2
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Without this fix, the build on AIX 7.2 with xlc fails in the ‘CCLD
temacs’ step with the diagnostic ‘ld: 0711-317 ERROR: Undefined
symbol: BC’.  This is because -lcurses does not define BC etc.
* configure.ac: When building terminfo.o, define
TERMINFO_DEFINES_BC if the library defines BC etc.
* src/terminfo.c (UP, BC, PC): Define depending on
TERMINFO_DEFINES_BC, not on TERMINFO.
---
 configure.ac   | 12 ++++++++++++
 src/terminfo.c |  6 +++---
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/configure.ac b/configure.ac
index 48e96529ff..07a11cc990 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4366,6 +4366,18 @@ AC_DEFUN
 if test $TERMINFO = yes; then
   AC_DEFINE(TERMINFO, 1, [Define to 1 if you use terminfo instead of termcap.])
   TERMCAP_OBJ=terminfo.o
+  AC_CACHE_CHECK([whether terminfo defines BC],
+    [emacs_cv_terminfo_defines_BC],
+    [OLD_LIBS=$LIBS
+     LIBS="$LIBS $LIBS_TERMCAP"
+     AC_LINK_IFELSE([AC_LANG_PROGRAM([[extern char *BC;]], [[return !*BC;]])],
+       [emacs_cv_terminfo_defines_BC=yes],
+       [emacs_cv_terminfo_defines_BC=no])
+     LIBS=$OLD_LIBS])
+  if test "$emacs_cv_terminfo_defines_BC" = yes; then
+    AC_DEFINE([TERMINFO_DEFINES_BC], 1, [Define to 1 if the
+      terminfo library defines the variables BC, PC, and UP.])
+  fi
 fi
 if test "X$LIBS_TERMCAP" = "X-lncurses"; then
   AC_DEFINE(USE_NCURSES, 1, [Define to 1 if you use ncurses.])
diff --git a/src/terminfo.c b/src/terminfo.c
index 15aff317f1..a9c9572bbb 100644
--- a/src/terminfo.c
+++ b/src/terminfo.c
@@ -23,10 +23,10 @@
 
 /* Define these variables that serve as global parameters to termcap,
    so that we do not need to conditionalize the places in Emacs
-   that set them.  But don't do that for terminfo, as that could
-   cause link errors when using -fno-common.  */
+   that set them.  But don't do that if terminfo defines them, as that
+   could cause link errors when using -fno-common.  */
 
-#if !TERMINFO
+#ifndef TERMINFO_DEFINES_BC
 char *UP, *BC, PC;
 #endif
 
-- 
2.27.0