Fix build with XPTI disabled (#18269)

There were two issues:
1) Setting `UR_ENABLE_TRACING` on the command line didn't work because
the SYCL code calling the FetchUR CMake files didn't use `CACHE`, also
fixed two other vars with the same problem.
2) XPTI SYCL tests were built even if XPTI wasn't built, causing a build
failure.

Also add two errors so the problem isn't discovered in a build failure.

---------

Signed-off-by: Sarnie, Nick <nick.sarnie@intel.com>

Author: sarnex

CMakeLists.txt

@@ -159,6 +159,13 @@ else()
     set(UR_DPCXX_DEPS "")
 endif()
 
+if(NOT UR_STANDALONE_BUILD AND
+    UR_ENABLE_TRACING AND
+    DEFINED LLVM_EXTERNAL_PROJECTS AND
+    (NOT "xpti" IN_LIST LLVM_EXTERNAL_PROJECTS OR NOT "xptifw" IN_LIST LLVM_EXTERNAL_PROJECTS))
+  message(FATAL_ERROR "UR_ENABLE_TRACING=ON but XPTI is not going to be built")
+endif()
+
 if(UR_ENABLE_TRACING)
     add_compile_definitions(UR_ENABLE_TRACING)