update boost install to include natvis files

[cmazakas]

Probe the interfaces sources of a library target and manually patch the target's properties such that the natvis files are installed and probably linked against for downstream targets.

Partially closes boostorg/unordered#307

For pure CMake builds, it seems like natvis files aren't loaded into the solution unless the end-target has the natvis file as a source file, which is only possible when they're carried transitively as an interface source.

This code enforces the largely existing idiom of using extra/boost_<lib>.natvis but we can maybe massage the regex to be more accommodating of other libraries like JSON and URL.