Fix Linux-specific compile errors in x11.cpp and print_handler_gtk.cpp

- cef_config.h: define CEF_X11 on Linux so cef_get_xdisplay() is declared
  (CEF_X11 guard was present in cef_types_linux.h but the macro was never set)
- x11.h: add <gtk/gtkx.h> for explicit GtkPlug/GtkSocket X11 declarations
- subprocess/CMakeLists.txt: add gtk+-unix-print-3.0 pkg-config module so
  its include dirs are available for <gtk/gtkunixprint.h>

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: linesight

src/client_handler/x11.h

@@ -6,6 +6,7 @@
 
 #include <X11/Xlib.h>
 #include <gtk/gtk.h>
+#include <gtk/gtkx.h>
 #include <gdk/gdkx.h>
 
 #include "include/cef_browser.h"

src/include/cef_config.h

@@ -37,4 +37,9 @@
 
 #define CEF_V8_ENABLE_SANDBOX 1
 
+// X11 windowing support (Linux only; required for cef_get_xdisplay and GtkPlug).
+#if defined(__linux__) && !defined(OS_CHROMEOS)
+#define CEF_X11 1
+#endif
+
 #endif  // CEF_INCLUDE_CEF_CONFIG_H_

src/subprocess/CMakeLists.txt

@@ -53,6 +53,7 @@ list(APPEND _app_sources ${_filtered_mml})
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
     find_package(PkgConfig REQUIRED)
     pkg_check_modules(GTK3 REQUIRED gtk+-3.0)
+    pkg_check_modules(GTK3_UNIXPRINT REQUIRED gtk+-unix-print-3.0)
 endif()
 
 add_library(cefpython_app STATIC ${_app_sources})
@@ -66,7 +67,10 @@ target_include_directories(cefpython_app PRIVATE
 )
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
-    target_include_directories(cefpython_app PRIVATE ${GTK3_INCLUDE_DIRS})
+    target_include_directories(cefpython_app PRIVATE
+        ${GTK3_INCLUDE_DIRS}
+        ${GTK3_UNIXPRINT_INCLUDE_DIRS}
+    )
 endif()
 
 if(WIN32)