Add cmake warning about setting UR_CONFORMANCE_TARGET_TRIPLES

Also document it in README

Author: aarongreig

README.md

@@ -112,6 +112,7 @@ List of options provided by CMake:
 | UR_USE_UBSAN | Enable UndefinedBehavior Sanitizer | ON/OFF | OFF |
 | UR_USE_MSAN | Enable MemorySanitizer (clang only) | ON/OFF | OFF |
 | UR_ENABLE_TRACING | Enable XPTI-based tracing layer | ON/OFF | OFF |
+| UR_CONFORMANCE_TARGET_TRIPLES | SYCL triples to build CTS device binaries for | Comma-separated list | spir64 |
 
 ### Additional make targets
 

test/conformance/CMakeLists.txt

@@ -64,9 +64,10 @@ if(DEFINED UR_DPCXX)
         "${CMAKE_CURRENT_BINARY_DIR}/device_binaries/")
     file(MAKE_DIRECTORY ${UR_CONFORMANCE_DEVICE_BINARIES_DIR})
 
-    if(DEFINED UR_CONFORMANCE_TARGET_TRIPLES)
+    if(NOT "${UR_CONFORMANCE_TARGET_TRIPLES}" STREQUAL "")
         string(REPLACE "," ";" TARGET_TRIPLES ${UR_CONFORMANCE_TARGET_TRIPLES})
     else()
+        message(WARNING "UR_CONFORMANCE_TARGET_TRIPLES wasn't set, defaulting to only generate spir64 device binaries")
         list(APPEND TARGET_TRIPLES "spir64")
     endif()