SDL: cmake: don't fail if Wayland is enabled but libdecor isn't found.

SDL Commits
1 
From 74aa4b635cb13d8d54eb15d84d37f687b3413311 Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Tue, 3 Aug 2021 03:10:14 -0400
Subject: [PATCH] cmake: don't fail if Wayland is enabled but libdecor isn't
 found.

Instead, just disable libdecor support.

This matches what the configure script does.
---
 cmake/sdlchecks.cmake | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/cmake/sdlchecks.cmake b/cmake/sdlchecks.cmake
index 8df592549e..dc58c56562 100644
--- a/cmake/sdlchecks.cmake
+++ b/cmake/sdlchecks.cmake
@@ -698,12 +698,13 @@ macro(CheckWayland)
       endif()
 
       if(WAYLAND_LIBDECOR)
-        pkg_check_modules(libdecor REQUIRED libdecor-0)
-
-        FindLibraryAndSONAME(decor-0)
-        set(SDL_VIDEO_DRIVER_WAYLAND_DYNAMIC_LIBDECOR "\"${DECOR_0_LIB_SONAME}\"")
-
-        add_definitions(-DHAVE_LIBDECOR_H)
+        pkg_check_modules(PKG_LIBDECOR libdecor-0)
+        if(PKG_LIBDECOR_FOUND)
+            set(HAVE_WAYLAND_LIBDECOR TRUE)
+            FindLibraryAndSONAME(decor-0)
+            set(SDL_VIDEO_DRIVER_WAYLAND_DYNAMIC_LIBDECOR "\"${DECOR_0_LIB_SONAME}\"")
+            add_definitions(-DHAVE_LIBDECOR_H)
+        endif()
       endif()
 
       set(SDL_VIDEO_DRIVER_WAYLAND 1)