Advertise support for Startup Notification when built with GTK

* etc/emacsclient.desktop, etc/emacsclient.desktop: Specify
StartupNotify=true.
* configure.ac (USE_STARTUP_NOTIFICATION): New variable, yes iff
HAVE_GTK.
* Makefile.in (install-etc): Remove StartupNotify=true from
etc/*.desktop unless USE_STARTUP_NOTIFICATION (bug#48783).

Makefile.in

@@ -100,6 +100,8 @@ FIND_DELETE = @FIND_DELETE@
 
 HAVE_NATIVE_COMP = @HAVE_NATIVE_COMP@
 
+USE_STARTUP_NOTIFICATION = @USE_STARTUP_NOTIFICATION@
+
 # ==================== Where To Install Things ====================
 
 # Location to install Emacs.app under GNUstep / macOS.
@@ -706,11 +708,15 @@ install-man:
 ## Note: emacs22 does not have all the resolutions.
 EMACS_ICON=emacs
 
+ifeq (${USE_STARTUP_NOTIFICATION},no)
+USE_STARTUP_NOTIFICATION_SED_CMD=-e "/^StartupNotify=true$$/d"
+endif
 install-etc:
 	umask 022; ${MKDIR_P} "$(DESTDIR)${desktopdir}"
 	tmp=etc/emacs.tmpdesktop; rm -f $${tmp}; \
 	sed -e "/^Exec=emacs/ s/emacs/${EMACS_NAME}/" \
 	  -e "/^Icon=emacs/ s/emacs/${EMACS_NAME}/" \
+	  $(USE_STARTUP_NOTIFICATION_SED_CMD) \
 	  ${srcdir}/etc/emacs.desktop > $${tmp}; \
 	${INSTALL_DATA} $${tmp} "$(DESTDIR)${desktopdir}/${EMACS_NAME}.desktop"; \
 	rm -f $${tmp}
@@ -718,6 +724,7 @@ install-etc:
 	client_name=`echo emacsclient | sed '$(TRANSFORM)'`${EXEEXT}; \
 	sed -e "/^Exec=emacsclient/ s|emacsclient|${bindir}/$${client_name}|" \
 	  -e "/^Icon=emacs/ s/emacs/${EMACS_NAME}/" \
+	  $(USE_STARTUP_NOTIFICATION_SED_CMD) \
 	  ${srcdir}/etc/emacsclient.desktop > $${tmp}; \
 	${INSTALL_DATA} $${tmp} "$(DESTDIR)${desktopdir}/$${client_name}.desktop"; \
 	rm -f $${tmp}

configure.ac

@@ -2892,6 +2892,11 @@ fi
 AC_SUBST(SETTINGS_CFLAGS)
 AC_SUBST(SETTINGS_LIBS)
 
+USE_STARTUP_NOTIFICATION=no
+if test "${HAVE_GTK}" = "yes"; then
+    USE_STARTUP_NOTIFICATION=yes
+fi
+AC_SUBST(USE_STARTUP_NOTIFICATION)
 
 dnl SELinux is available for GNU/Linux only.
 HAVE_LIBSELINUX=no

etc/NEWS

@@ -91,6 +91,12 @@ proper pty support that Emacs needs.
 
 * Startup Changes in Emacs 28.1
 
+---
+** In GTK builds, Emacs now supports startup notification.
+This means that Emacs won't steal keyboard focus upon startup
+(when started via the Desktop) if the user is typing into another
+application.
+
 ** Emacs can support 24-bit color TTY without terminfo database.
 If your text-mode terminal supports 24-bit true color, but your system
 lacks the terminfo database, you can instruct Emacs to support 24-bit

etc/emacs.desktop

@@ -8,5 +8,6 @@ Icon=emacs
 Type=Application
 Terminal=false
 Categories=Development;TextEditor;
+StartupNotify=true
 StartupWMClass=Emacs
 Keywords=Text;Editor;

etc/emacsclient.desktop

@@ -8,5 +8,6 @@ Icon=emacs
 Type=Application
 Terminal=false
 Categories=Development;TextEditor;
+StartupNotify=true
 StartupWMClass=Emacsd
 Keywords=Text;Editor;