[libunwind] Enable LLVM_ENABLE_WARNINGS when building standalone out of tree

[mstorsjo]

Summary

When built within the llvm runtimes directory, the runtimes CMakeLists.txt adds the same.

This would have helped catch the issue behind D56905 over a year ago.

libunwind/trunk/CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
   else()
     message(WARNING "UNSUPPORTED LIBUNWIND CONFIGURATION DETECTED: "
                     "llvm-config not found and LLVM_MAIN_SRC_DIR not defined. "
                     "Reconfigure with -DLLVM_CONFIG=path/to/llvm-config "
                     "or -DLLVM_PATH=path/to/llvm-source-root.")
   endif()
 
   if (EXISTS ${LLVM_CMAKE_PATH})
+    # Enable warnings, otherwise -w gets added to the cflags by HandleLLVMOptions.
+    set(LLVM_ENABLE_WARNINGS ON)
     list(APPEND CMAKE_MODULE_PATH "${LLVM_CMAKE_PATH}")
     include("${LLVM_CMAKE_PATH}/AddLLVM.cmake")
     include("${LLVM_CMAKE_PATH}/HandleLLVMOptions.cmake")
   else()
     message(WARNING "Not found: ${LLVM_CMAKE_PATH}")
   endif()
 
   set(PACKAGE_NAME libunwind)