Set GAP_KERNEL_API_VERSION in configure.ac, use it to modify GAPARCH

[fingolfin]

Currently, we #define GAP_KERNEL_API_VERSION in src/modules.h, with the idea being that we increment it when there are kernel API / ABI changes that cause incompatibilities.

We should consider changing this so that we set the API/ABI version in configure.ac, and then propagate this to src/modules.h. This has the advantage that we can, in the future, use this to adapt how libgap shared library is built and named. Moreover, we can use it to modify the GAPARCH string; this then will cause kernel extensions in packages to be built in a different directory, which in turn is useful if one is using several mutually incompatible GAP versions in parallel.

I can't think of any drawbacks right now.

[fingolfin]

Done in PR #2873