avoid using multiline comment

Because following error reported by clang in OSX:

```log
clang: error: no such file or directory: 'Unsupported'
clang: error: no such file or directory: 'if'
clang: error: no such file or directory: 'flag(ASIO)'
clang: error: no such file or directory: 'include-dirs:'
clang: error: no such file or directory: 'c-sources:'
clang: error: no such file or directory: 'cc-options:'
```