emacs-27 585997d: Fix broken build on AIX 7.2

[Paul Eggert]

branch: emacs-27
commit 585997d05adde5a3510ce1221f9e8dd60407ce30
Author: Paul Eggert <eggert@cs.ucla.edu>
Commit: Paul Eggert <eggert@cs.ucla.edu>

    Fix broken build on AIX 7.2
    
    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 48e9652..89b0785 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4366,6 +4366,18 @@ TERMCAP_OBJ=tparam.o
 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 $LIBS_TERMCAP library defines BC],
+    [emacs_cv_terminfo_defines_BC],
+    [OLD_LIBS=$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 15aff31..a9c9572 100644
--- a/src/terminfo.c
+++ b/src/terminfo.c
@@ -23,10 +23,10 @@ along with GNU Emacs.  If not, see 
<https://www.gnu.org/licenses/>.  */
 
 /* 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