Add major/minor version number macros

[ryanofsky]

It would probably useful to add major/minor version numbers to libmultiprocess to help provide clearer build errors when an incompatible version is used.

• Bumping minor version number when new features are added could have been used to provide clearer "Installed libmultiprocess version is too old" error in error: 'Proxy' has no member named 'include' #44 .

• Bumping major number number when potential backwards incompatible changes are made could have been used to provided clearer "Installed libmultiprocess version is too new" error or warning in Build fail in bitcoin core on macOS #52.

[promag]

Concept ACK.