cmake: Print a warning if the libdecor development library is missing

libdecor is required for window decorations on Wayland when the toplevel decoration protocol is not supported, such as on GNOME and Weston. Warn if the development library can't be found, unless it was explicitly disabled.

cmake/sdlchecks.cmake

@@ -758,7 +758,9 @@ macro(CheckWayland)
             sdl_link_dependency(libdecor INCLUDES $<TARGET_PROPERTY:PkgConfig::PC_LIBDECOR,INTERFACE_INCLUDE_DIRECTORIES>)
           else()
             sdl_link_dependency(libdecor LIBS PkgConfig::PC_LIBDECOR PKG_CONFIG_PREFIX PC_LIBDECOR PKG_CONFIG_SPECS ${LibDecor_PKG_CONFIG_SPEC})
-            endif()
+          endif()
+        else()
+          message(WARNING "Unable to find the libdecor development library (required for window decorations on GNOME and Weston)")
         endif()
       endif()