freebsd clang support

[crabtw]

• On FreeBSD 9.0, the builtin clang is a modified version.
  The output of --version is "FreeBSD clang version 3.0 (branches/release_30 142614) 20111021" that is not the same as ordinary clang.
  I remove the text before "version 3.0" to get the correct number.
• When using clang to compile rust_shape.cpp, it shows a error message that indicates "alignof" is a keyword in C++11. I add "-Wno-c++11-compat" to suppress the error.
• Clang needs libunwind to get unwind support. Some types have been defined in the library so I add a condition to skip them

[brson]

Integrated. Thanks!