[llvm-bugs] [Bug 41866] New: Building with -D LLVM_ENABLE_PROJECTS='clang; lldb' on macOS errors for missing dependency target "cxx"

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41866

            Bug ID: 41866
           Summary: Building with -D LLVM_ENABLE_PROJECTS='clang;lldb' on
                    macOS errors for missing dependency target "cxx"
           Product: Build scripts
           Version: trunk
          Hardware: Macintosh
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: cmake
          Assignee: jryans at gmail.com
          Reporter: jryans at gmail.com
                CC: jdevlieghere at apple.com, llvm-bugs at lists.llvm.org,
                    stefan.graenitz at gmail.com

I typically try to limit the part of LLVM I build a config such as

```
-D LLVM_ENABLE_PROJECTS='clang;lldb'
```

but this particular combo currently fails on macOS with many errors like:

CMake Error at cmake/modules/AddLLVM.cmake:1391 (add_dependencies):
  The dependency target "cxx" of target "check-all" does not exist.
Call Stack (most recent call first):
  CMakeLists.txt:994 (add_lit_target)

CMake Error at /Users/jryans/Projects/llvm-project/lldb/CMakeLists.txt:137
(add_dependencies):
  The dependency target "cxx" of target "lldb-test-deps" does not exist.

CMake Error at /Users/jryans/Projects/llvm-project/lldb/test/CMakeLists.txt:13
(add_dependencies):
  The dependency target "cxx" of target "check-lldb-single" does not exist.
Call Stack (most recent call first):
  /Users/jryans/Projects/llvm-project/lldb/test/CMakeLists.txt:108
(add_python_test_target)

[... many more lines about "cxx" ...]

The root cause here is that LLDB requires libcxx on macOS, but the errors above
don't make that very clear, at least for those new to LLVM. I think this could
be clarified with two changes:

1. Check whether `libcxx` is enabled as a project, and output a specific error
about this if not
2. Update the LLDB docs to suggest a valid value of `LLVM_ENABLE_PROJECTS`

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