Simplify CMake by using LLVM Cmake config files

[DaanDeMeyer]

Simplifies the CMake build even further by using the LLVM CMake config files instead of implementing the logic ourselves.

I couldn't use the LLVM CMake config files in cquery because they aren't bundled with the LLVM Windows archive just like the LLVM libraries but since you use the LLVM libraries it's already impossible to use ccls with the Windows archive which means we might as well assume the LLVM CMake config files are available.

I verified ccls builds with these changes but I don't use it myself so you might want to check if everything still works as expected before merging this.

[MaskRay]

Thanks for your help!

by using the LLVM CMake config files instead of implementing the logic ourselves.

Which llvm cmake files does it read?

-I for clang/llvm headers isn't specified after the simplification.

LLVM=$(realpath ~/llvm)
cmake -H. -Bbuild -G Ninja -DCMAKE_PREFIX_PATH="$LLVM/Release;$LLVM/Release/tools/clang;$LLVM;$LLVM/tools/clang"

ninja -C build:

../src/serializer.hh:20:10: fatal error: 'llvm/Support/Compiler.h' file not found
#include <llvm/Support/Compiler.h>

[DaanDeMeyer]

Did you ninja -C build install after building LLVM? I think the LLVM CMake config files assume you've installed LLVM and don't work directly from the build directory. That might be another downside of this approach.

The config files themselves should be located in <prefix>/lib/cmake/llvm after installing.

[MaskRay]

@madscientist Thanks. Changed to

target_include_directories(ccls PRIVATE src)
target_include_directories(ccls SYSTEM PRIVATE
  third_party ${LLVM_INCLUDE_DIRS} ${CLANG_INCLUDE_DIRS})

I'm surprised target_include_directories is needed at all. The include directories should get added automatically by target_link_libraries.

@DaanDeMeyer I don't understand how target_include_directories target_link_libraries work, but ${LLVM_INCLUDE_DIRS} ${CLANG_INCLUDE_DIRS} are required for this case.