Updates CheckLibUDev() to match main, fixing missing HAVE_LIBUDEV

cmake/sdlchecks.cmake

@@ -1356,12 +1356,13 @@ endmacro()
 
 macro(CheckLibUDev)
   if(SDL_LIBUDEV)
-    check_include_file("libudev.h" have_libudev_header)
-    if(have_libudev_header)
+    check_include_file("libudev.h" HAVE_LIBUDEV_HEADER)
+    if(HAVE_LIBUDEV_HEADER)
       set(HAVE_LIBUDEV_H TRUE)
       FindLibraryAndSONAME(udev)
       if(UDEV_LIB_SONAME)
         set(SDL_UDEV_DYNAMIC "\"${UDEV_LIB_SONAME}\"")
+        set(HAVE_LIBUDEV TRUE)
       endif()
     endif()
   endif()