fix(cefpython.pyx): guard X11 error-handler install at compile time

Wrapping the InstallX11ErrorHandlers() call with a runtime
sys.platform check is not enough — Cython still semantically
analyses the right-hand side, and _g_linux_wayland_mode is only
defined when window_utils_linux.pyx is included in the build
(Linux only).  macOS / Windows builds therefore fail with
"undeclared name not builtin: _g_linux_wayland_mode" during
Cython compilation.

Use the existing IF UNAME_SYSNAME == "Linux" compile-time
directive (same pattern used for the other Linux-only blocks in
this file at lines 580, 845, 990, 1021) so the body is excluded
on non-Linux hosts.  The redundant runtime sys.platform check
inside it is dropped.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: linesight

src/cefpython.pyx

@@ -699,8 +699,13 @@ def Initialize(applicationSettings=None, commandLineSwitches=None, **kwargs):
             Debug("CefInitialize() WARNING: OnContextInitialized not received"
                   " within 30 seconds")
 
-    if sys.platform.startswith("linux") and not _g_linux_wayland_mode:
-        WindowUtils.InstallX11ErrorHandlers()
+    # Compile-time Linux guard: _g_linux_wayland_mode is defined in
+    # window_utils_linux.pyx, which is only included in the Linux build,
+    # so this whole block must be excluded on Windows/macOS or Cython
+    # fails with "undeclared name not builtin: _g_linux_wayland_mode".
+    IF UNAME_SYSNAME == "Linux":
+        if not _g_linux_wayland_mode:
+            WindowUtils.InstallX11ErrorHandlers()
 
 
     return ret