Use llvm-objcopy if using clang and it's available

This works around a weird error I was getting when trying to link a
translation unit that included Internal.h with libconflict-set-static.a
with clang + gnu objcopy

CMakeLists.txt

@@ -36,6 +36,12 @@ add_compile_options(-fdata-sections -ffunction-sections -Wswitch-enum
 
 if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
   add_link_options("-Wno-unused-command-line-argument")
+  find_program(LLVM_OBJCOPY llvm-objcopy)
+  if(LLVM_OBJCOPY)
+    set(CMAKE_OBJCOPY
+        ${LLVM_OBJCOPY}
+        CACHE FILEPATH "path to objcopy binary" FORCE)
+  endif()
 endif()
 
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")