cmake: delete SYSTEM_CLANG and auto-download mechanism

[MaskRay]

Notably, delete SYSTEM_CLANG which downloads prebuilt clang+llvm archives from releases.llvm.org

[MaskRay]

This does not work currently. I must have missed something:

% cmake -H. -Bbuild -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_COMPILER=~/llvm/Stable/bin/clang++ -DCMAKE_PREFIX_PATH="$LLVM/Release;$LLVM/Release/tools/clang;$LLVM;$LLVM/tools/clang"
-- Could NOT find Clang (missing: Clang_LIBRARY Clang_INCLUDE_DIR LLVM_INCLUDE_DIR LLVM_BUILD_INCLUDE_DIR) (found suitable version "8.0.0", minimum required is "7.0.0")
-- Configuring done
CMake Error at CMakeLists.txt:12 (add_executable):
  Target "ccls" links to target "Clang::Clang" but the target was not found.
  Perhaps a find_package() call is missing for an IMPORTED target, or an
  ALIAS target is missing?


CMake Error at CMakeLists.txt:12 (add_executable):
  Target "ccls" links to target "Clang::Clang" but the target was not found.
  Perhaps a find_package() call is missing for an IMPORTED target, or an
  ALIAS target is missing?

@DaanDeMeyer I'm sorry to bother you but would you like to help?

[DaanDeMeyer]

Your changes work for me (Arch Linux, clang+llvm packages installed). Most likely your CMAKE_PREFIX_PATH isn't entirely correct.

Make sure the lib and bin folders with the clang libraries and binaries are in a direct subdirectory of any directory in CMAKE_PREFIX_PATH