Standalone libclc build fails on 19.1.0

[satmandu]

Attempting a standalone libclc build on Linux after building llvm 19.1.0 with libclc built-in (which itself appears to be incomplete because mesa complains about a missing libclc.):

-- Found LibXml2: /usr/local/lib64/libxml2.so (found version "2.13.3")
-- Linker detection: GNU ld
-- libclc LLVM version: 19.1.0
CMake Error at /usr/local/lib64/cmake/llvm/AddLLVM.cmake:1029 (add_executable):
  add_executable cannot create target "prepare_builtins" because an imported
  target with the same name already exists.
Call Stack (most recent call first):
  CMakeLists.txt:170 (add_llvm_executable)


CMake Error at /usr/local/lib64/cmake/llvm/LLVM-Config.cmake:95 (target_link_libraries):
  IMPORTED library can only be used with the INTERFACE keyword of
  target_link_libraries
Call Stack (most recent call first):
  /usr/local/lib64/cmake/llvm/AddLLVM.cmake:1089 (llvm_config)
  CMakeLists.txt:170 (add_llvm_executable)


CMake Error at /usr/local/lib64/cmake/llvm/AddLLVM.cmake:1106 (target_link_libraries):
  IMPORTED library can only be used with the INTERFACE keyword of
  target_link_libraries
Call Stack (most recent call first):
  CMakeLists.txt:170 (add_llvm_executable)


CMake Error at CMakeLists.txt:177 (target_compile_definitions):
  target_compile_definitions may only set INTERFACE properties on IMPORTED
  targets


-- Found Python3: /usr/local/bin/python3.12 (found version "3.12.6") found components: Interpreter
-- libclc target 'amdgcn--' is enabled

Originally posted by @satmandu in #101154 (comment)

[satmandu]

More specifically, with llvm-spirv available on my system, spirv64-mesa3d-.spv and spirv-mesa3d-.spv never get built by the default LLVM build, but they would get built by the standalone libclc build, which no longer works.

[satmandu]

I figured this out. The standalone build only works if I haven't built LLVM with libclc. If I have, it refuses to build.