[cmake] validate CHECK_SYCL_ALL more thoroughly during configuration (intel#1321)

I had a bit of trouble working out why lit was failing to start after
configuration had succceeded. Turns out I was passing a partial tuple
to cmake `-DCHECK_SYCL_ALL` due to a typo in a shell variable.

This patch verifies that both parts of each tuple in the
`CHECK_SYCL_ALL` variable are nonempty and terminates configuration with
an error message otherwise, which should make it a bit more obvious
what's happening.

Author: ldrumm

SYCL/CMakeLists.txt

@@ -15,6 +15,13 @@ if(CHECK_SYCL_ALL)
     list (GET BE_LIST 0 TARGET_BE)
     list (GET BE_LIST 1 TARGET_DEVICES)
 
+    if ("${TARGET_BE}" STREQUAL "")
+      message(FATAL_ERROR
+        "invalid empty target backend specification in CHECK_SYCL_ALL")
+    elseif("${TARGET_DEVICES}" STREQUAL "")
+      message(FATAL_ERROR
+        "invalid empty target device specification in CHECK_SYCL_ALL")
+    endif()
     message("Run on ${TARGET_DEVICES} for ${TARGET_BE}")
 
     string(REPLACE "," "_" TARGET check-sycl-${TARGET_BE}-${TARGET_DEVICES})