Build warnings on Windows

[ahoarau]

Building coal on windows will issue those messages:

[84/171] Building CXX object python-nb\CMakeFiles\coal_pywrap_nb.dir\aabb.cc.obj                                                                                                                                                                              
Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately. For example:
- add -D_WIN32_WINNT=0x0601 to the compiler command line; or
- add _WIN32_WINNT=0x0601 to your project's Preprocessor Definitions.
Assuming _WIN32_WINNT=0x0601 (i.e. Windows 7 target).
[85/171] Building CXX object python-nb\CMakeFiles\coal_pywrap_nb.dir\collision-geometries.cc.obj
Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately. For example:
- add -D_WIN32_WINNT=0x0601 to the compiler command line; or
- add _WIN32_WINNT=0x0601 to your project's Preprocessor Definitions.
Assuming _WIN32_WINNT=0x0601 (i.e. Windows 7 target).
[86/171] Building CXX object python-nb\CMakeFiles\coal_pywrap_nb.dir\distance.cc.obj
Please define _WIN32_WINNT or _WIN32_WINDOWS appropriately. For example:
- add -D_WIN32_WINNT=0x0601 to the compiler command line; or
- add _WIN32_WINNT=0x0601 to your project's Preprocessor Definitions.
Assuming _WIN32_WINNT=0x0601 (i.e. Windows 7 target).

I failed to understand which library/compile flags triggered this, but it seem to be related with some library using an old windows api/sdk.
I wouldn't add add_definitions(_WIN32_WINNT=0x0601) as suggested, that would hide the actual issue.

[ahoarau]

@jorisv The issue is still there unfortunately. I think it's from an external lib.

[ahoarau]

Same issue on pinocchio, it comes from Boost::serialization or Boost::asio