lld: copy unwind only when building from source

[ArchangeGabriel]

Currenty, building with a prebuilt lld results in:

CMake Error at external/llvm/llvm_deps.cmake:50 (file):
  file COPY cannot find
  "/build/intel-graphics-compiler/src/intel-graphics-compiler-igc-1.0.8744/IGC/DEFAULT_IGC_LLVM_SOURCES_DIR-NOTFOUND/libunwind/include/mach-o/compact_unwind_encoding.h":
  No such file or directory.
Call Stack (most recent call first):
  IGC/CMakeLists.txt:1252 (include)

I think this code should only be run when building from source as the comment suggest, so I (hopefully) moved it to the right place.

[ArchangeGabriel]

(At least it allowed to build in my case, but I cannot check yours)

[pszymich]

Hi @ArchangeGabriel, was this pull-request supposed to fix #209?

[ArchangeGabriel]

No, it’s a different issue. If you try to build with -DIGC_OPTION__LLD_MODE=Prebuilds without this patch it won’t work.

[ArchangeGabriel]

Included in 4369c97.