Add package configuration file to CPP version.

[wnts]

Resolves #901.

[wnts]

This still requires some extra work, which I will only perform if there is interest in this PR:

1. Right now, the exported target contains by default the Boost::system (for boost adaptors and X3 parser support) and zlib dependencies, and they are always searched for by the package config file. Some users don't use these features so they should be optional (I'm thinking about treating them as components in the find_package call).

2. Set correct MSGPACK_* preprocessor macros on the exported target:

• MSGPACK_USE_X3_PARSE: Can be set if this component is requested by user (see 1. above)
• MSGPACK_USE_CPP03 : Can be set from information gathered by compiler inspection to determine if C++11 or later is supported (MSGPACK_USE_CPP03)

3. ... Suggestions ?