Fix conflict between variable and macro name

[demiquartz]

Fix conflict between variable and macro name when building on Msys2 / MinGW64.

The errors that occurred are:

libtins/src/ipv6.cpp:377:35: error: expected primary-expression before 'struct'
  377 |         link_addr.sin6_scope_id = interface.id();
      |                                   ^~~~~~~~~
ninja: build stopped: subcommand failed.

This change defines WIN32_LEAN_AND_MEAN and prevents interface macros from being defined.

[mfontanini]

Thanks and sorry for the delay!