x11vulkan: Use the correct SONAME of libX11-xcb.so.1 on Linux, etc.

On most Unix platforms supported by SDL, the canonical name used to load
a library at runtime includes its ABI major version, and the name
without a version is not guaranteed to exist on non-developer systems.
libX11-xcb.so.1 is correct on Linux, and probably on other Unix
platforms like FreeBSD.

A notable exception is OpenBSD, which apparently does not use
ABI-suffixed names, so continue to use libX11-xcb.so there.

Signed-off-by: Simon McVittie <smcv@collabora.com>

include/SDL3/SDL_hints.h

@@ -3979,7 +3979,7 @@ extern "C" {
 /**
  * Specify the XCB library to load for the X11 driver.
  *
- * This defaults to "libX11-xcb.so"
+ * The default is platform-specific, often "libX11-xcb.so.1".
  *
  * This hint should be set before initializing the video subsystem.
  *

src/video/x11/SDL_x11vulkan.c

@@ -33,8 +33,10 @@
 
 #ifdef SDL_PLATFORM_OPENBSD
 #define DEFAULT_VULKAN "libvulkan.so"
+#define DEFAULT_X11_XCB "libX11-xcb.so"
 #else
 #define DEFAULT_VULKAN "libvulkan.so.1"
+#define DEFAULT_X11_XCB "libX11-xcb.so.1"
 #endif
 
 /*
@@ -109,7 +111,7 @@ bool X11_Vulkan_LoadLibrary(SDL_VideoDevice *_this, const char *path)
     } else {
         const char *libX11XCBLibraryName = SDL_GetHint(SDL_HINT_X11_XCB_LIBRARY);
         if (!libX11XCBLibraryName || !*libX11XCBLibraryName) {
-            libX11XCBLibraryName = "libX11-xcb.so";
+            libX11XCBLibraryName = DEFAULT_X11_XCB;
         }
         videoData->vulkan_xlib_xcb_library = SDL_LoadObject(libX11XCBLibraryName);
         if (!videoData->vulkan_xlib_xcb_library) {