[libclc] [libclc] Give a helpful error when an unknown target is requested (PR #111528)

[Fraser Cormack via cfe-commits]

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@@ -151,6 +147,17 @@ if( "spirv-mesa3d-" IN_LIST LIBCLC_TARGETS_TO_BUILD OR "spirv64-mesa3d-" IN_LIST
   endif()
 endif()
 
+if( LIBCLC_TARGETS_TO_BUILD STREQUAL "all" )
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frasercrmck wrote:

Right yeah, because `spirv-mesa3d-` is only added to `ALL` if the tool is available, so would always flag as an unknown target. That makes sense, thanks.

https://github.com/llvm/llvm-project/pull/111528