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 #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 #52.

[promag]

Concept ACK.