Export version for CMake

[SunBlack]

This allows requesting a minimum version.

E.g. when you have in your CMake:

find_package(E57Format 3.2.1 CONFIG REQUIRED)

And you have build this lib with this PR but the version number is still 3.2.0, you are getting

CMake Error at CMakeLists.txt:5 (find_package):
  Could not find a configuration file for package "E57Format" that is
  compatible with requested version "3.2.1".

  The following configuration files were considered but not accepted:

    .../lib/cmake/E57Format/e57format-config.cmake, version: 3.2.0

As there is a note that the changes to the legacy enums (which were changed in 2.x => 3.x) will only be removed with 4.x, I assume that SameMajorVersion is the best choice. For details see documentation (and example below)

[asmaloney]

Thanks!