message: Only shell out to llvm-config if the configure step found it
diff: @@ -304,9 +304,12 @@ def build(bld):
  lib.append('ncurses')
  if bld.env['use_system_clang']:
  + if bld.env['llvm_config']:
  rpath = str(subprocess.check_output(
  [bld.env['llvm_config'], '--libdir'],
  stderr=subprocess.STDOUT).decode()).strip()
  + else:
  + rpath = []
  # Use CXX set by --check-cxx-compiler if it is "clang".
  # See https://github.com/jacobdufault/cquery/issues/237
  