Fix build failure on Debian bullseye from last commit.

Jacob reports that bullseye objected to the change from
G_APPLICATION_FLAGS_NONE to G_APPLICATION_DEFAULT_FLAGS, on the
grounds that it only has the former defined. Sigh. Added a cmake
check.

cmake/cmake.h.in

@@ -48,6 +48,7 @@
 #cmakedefine01 HAVE_BINARY_SETPGRP
 #cmakedefine01 HAVE_PANGO_FONT_FAMILY_IS_MONOSPACE
 #cmakedefine01 HAVE_PANGO_FONT_MAP_LIST_FAMILIES
+#cmakedefine01 HAVE_G_APPLICATION_DEFAULT_FLAGS
 
 #cmakedefine01 HAVE_AES_NI
 #cmakedefine01 HAVE_SHA_NI

cmake/gtk.cmake

@@ -84,6 +84,10 @@ if(GTK_FOUND)
       ${HAVE_PANGO_FONT_FAMILY_IS_MONOSPACE} PARENT_SCOPE)
     set(HAVE_PANGO_FONT_MAP_LIST_FAMILIES
       ${HAVE_PANGO_FONT_MAP_LIST_FAMILIES} PARENT_SCOPE)
+    check_c_source_compiles("
+      #include <gtk/gtk.h>
+      int f = G_APPLICATION_DEFAULT_FLAGS;
+      int main(void) {}" HAVE_G_APPLICATION_DEFAULT_FLAGS)
   endfunction()
   pango_check_subscope()
 endif()

unix/gtkcompat.h

@@ -218,3 +218,7 @@
     gdk_cursor_new_for_display(gdk_display_get_default(), cur)
 
 #endif /* 3.0 */
+
+#if !HAVE_G_APPLICATION_DEFAULT_FLAGS
+#define G_APPLICATION_DEFAULT_FLAGS G_APPLICATION_FLAGS_NONE
+#endif

unix/main-gtk-application.c

@@ -79,6 +79,7 @@ https://wiki.gnome.org/Projects/GTK%2B/OSX/Bundling has some links.
 
 #include "putty.h"
 #include "gtkmisc.h"
+#include "gtkcompat.h"
 
 char *x_get_default(const char *key) { return NULL; }