Avoid installing external LLVM dylibs

src/bootstrap/dist.rs

@@ -2335,6 +2335,22 @@ fn maybe_install_llvm(builder: &Builder<'_>, target: TargetSelection, dst_libdir
         return;
     }
 
+    if let Some(config) = builder.config.target_config.get(&target) {
+        if config.llvm_config.is_some() {
+            // If the LLVM was externally provided, then we don't currently copy
+            // artifacts into the sysroot. This is not necessarily the right
+            // choice (in particular, it will require the LLVM dylib to be in
+            // the linker's load path at runtime), but the common use case for
+            // external LLVMs is distribution provided LLVMs, and in that case
+            // they're usually in the standard search path (e.g., /usr/lib) and
+            // copying them here is going to cause problems as we may end up
+            // with the wrong files and isn't what distributions want.
+            //
+            // This behavior may be revisited in the future though.
+            return;
+        }
+    }
+
     // On macOS, rustc (and LLVM tools) link to an unversioned libLLVM.dylib
     // instead of libLLVM-11-rust-....dylib, as on linux. It's not entirely
     // clear why this is the case, though. llvm-config will emit the versioned