[SYCL] Fix UR standalone build (#18829)

Fixes issue reported
[here](#18599 (comment)).

Signed-off-by: Sarnie, Nick <[email protected]>

Author: sarnex

sycl/cmake/modules/FetchEmhash.cmake

@@ -1,4 +1,7 @@
 # Finds or fetches emhash.
+if(DEFINED SYCL_EMHASH_DIR OR DEFINED EMHASH_SYS_LOC)
+  return()
+endif()
 find_file(EMHASH_SYS_LOC "hash_table8.hpp" PATH_SUFFIXES "emhash")
 if(NOT EMHASH_SYS_LOC)
   set(EMHASH_REPO https://github.com/ktprime/emhash)

xptifw/src/CMakeLists.txt

@@ -1,9 +1,11 @@
 # Fetch third party headers
 include(FetchContent)
 
-# The sycl target should download or find emhash.
+# The sycl target should download or find emhash but may not
+# for standalone builds.
 if (NOT SYCL_EMHASH_DIR)
-  message(FATAL_ERROR "emhash headers not found")
+  list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/../../sycl/cmake/modules")
+  include(FetchEmhash)
 endif()
 
 set(PARALLEL_HASHMAP_REPO https://github.com/greg7mdp/parallel-hashmap.git)