add splat-version.h to define the library release version information

[hobu]

As mentioned in #48 (comment), this PR adds library version information taken from the project CMake defintion and makes it available via the splat-version.h public header.

An example output might be something like this:

#pragma once


/*
 * version settings
 */
#define SPZ_VERSION_MAJOR 1
#define SPZ_VERSION_MINOR 1
#define SPZ_VERSION_PATCH 0

This PR also adds GDAL's COMPUTE_VERSION macro for convenient version comparisons. This is becoming important due to updates like #36 requiring client code to be version sensitive.

#if SPZ_VERSION_NUM >= SPZ_COMPUTE_VERSION(1,1,0)
///
#endif