configure warns about an implicit function declaration for XOpenDisplay,
which in turn triggers a QA warning in Gentoo's package manager.

The relevant snippet from configure.log is:

configure:14158: checking whether X on GNU/Linux needs -b to link
configure:14175: x86_64-pc-linux-gnu-gcc -o conftest  -march=native -ggdb -O2 -pipe     -Wl,-O1 -Wl,--as-needed  conftest.c -lX11   >&5 
conftest.c: In function 'main':
conftest.c:125:1: warning: implicit declaration of function 'XOpenDisplay' [-Wimplicit-function-declaration]
  125 | XOpenDisplay ("foo");
      | ^~~~~~~~~~~~
configure:14175: $? = 0
configure:14223: result: no

Attached patch fixes the problem for me. Is it OK to merge it to the
emacs-29 branch? (In master, the test in question was removed altogether
by commit 6c1413d5ef0d1fea639b0d8c83a0c0065d99359b.)


In GNU Emacs 29.2 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.18.0) of 2024-01-19 built on localhost
Windowing system distributor 'The X.Org Foundation', version 11.0.12101011
System Description: Gentoo Linux

Configured using:
 'configure --prefix=/usr --build=x86_64-pc-linux-gnu
 --host=x86_64-pc-linux-gnu --mandir=/usr/share/man
 --infodir=/usr/share/info --datadir=/usr/share --sysconfdir=/etc
 --localstatedir=/var/lib --datarootdir=/usr/share
 --disable-silent-rules --docdir=/usr/share/doc/emacs-29.2
 --htmldir=/usr/share/doc/emacs-29.2/html --libdir=/usr/lib64
 --program-suffix=-emacs-29 --includedir=/usr/include/emacs-29
 --infodir=/usr/share/info/emacs-29 --localstatedir=/var
 --enable-locallisppath=/etc/emacs:/usr/share/emacs/site-lisp
 --without-compress-install --without-hesiod --without-pop
 --with-file-notification=inotify --with-pdumper --enable-acl
 --with-dbus --with-modules --with-gameuser=:gamestat --with-libgmp
 --with-gpm --without-native-compilation --without-json
 --without-kerberos --without-kerberos5 --with-lcms2 --with-xml2
 --without-mailutils --without-selinux --with-small-ja-dic
 --without-sqlite3 --with-gnutls --without-libsystemd --with-threads
 --without-tree-sitter --without-wide-int --with-sound=alsa --with-zlib
 --with-x --without-pgtk --without-ns --without-gconf --with-gsettings
 --without-toolkit-scroll-bars --with-xpm --with-xft --with-cairo
 --with-harfbuzz --with-libotf --with-m17n-flt --with-x-toolkit=lucid
 --with-xaw3d --with-gif --with-jpeg --with-png --with-rsvg --with-tiff
 --without-webp --with-imagemagick --with-dumping=pdumper
 'CFLAGS=-march=native -ggdb -O2 -pipe' 'LDFLAGS=-Wl,-O1
 -Wl,--as-needed''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ
IMAGEMAGICK JPEG LCMS2 LIBOTF LIBXML2 M17N_FLT MODULES NOTIFY INOTIFY
PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF X11 XAW3D XDBE XIM XINPUT2
XPM LUCID ZLIB

Important settings:
  value of $LC_CTYPE: en_GB.UTF-8
  value of $LC_TIME: en_GB.UTF-8
  value of $LANG: POSIX
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message mailcap yank-media puny dired
dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg rfc6068
epg-config gnus-util text-property-search time-date subr-x mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
cl-loaddefs cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util
mail-prsvr mail-utils rmc iso-transl tooltip cconv eldoc paren electric
uniquify ediff-hook vc-hooks lisp-float-type elisp-mode mwheel
term/x-win x-win term/common-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list replace newcomment text-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch easymenu
timer select scroll-bar mouse jit-lock font-lock syntax font-core
term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine cham georgian utf-8-lang misc-lang vietnamese
tibetan thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo x-toolkit
xinput2 x multi-tty make-network-process emacs)

Memory information:
((conses 16 37361 8954)
 (symbols 48 5129 0)
 (strings 32 14103 1583)
 (string-bytes 1 387374)
 (vectors 16 9311)
 (vector-slots 8 148873 16424)
 (floats 8 35 21)
 (intervals 56 348 0)
 (buffers 976 12))