[libclc] [llvm] [libclc] Support LLVM_ENABLE_RUNTIMES when building (PR #141574)

[Romaric Jodin via llvm-commits]

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@@ -204,6 +204,13 @@ if ("flang-rt" IN_LIST LLVM_ENABLE_RUNTIMES)
   endif ()
 endif ()
 
+if ("libclc" IN_LIST LLVM_ENABLE_PROJECTS)
+  message(WARNING "Using LLVM_ENABLE_PROJECTS=libclc is deprecated now, and will "
+    "become a fatal error in the LLVM 21 release.  Please use "
+    "-DLLVM_ENABLE_RUNTIMES=libclc or see the instructions at "
+    "https://libclc.llvm.org/ for building the runtimes.")
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rjodinchr wrote:

hum, just making that change in `libclc` is not enough.
I get that kind of error (building from `clvk` thus the `external/clspv` prefix):
```
ninja: error: 'external/clspv/cmake/prepare-clspv64--.bc', needed by 'external/clspv/include/clspv/clspv64_builtin_library.h', missing and no known rule to make it
```
It feels like the targets do not exist. I will dig into that a bit more in 2 weeks.

https://github.com/llvm/llvm-project/pull/141574