build: Remove the ARCTICDB_USING_CONDA and the ARCTICDB_USING_STATIC_XXHASH constant definitions and use CMake commands instead.

[jjerphan]

The ARCTICDB_USING_CONDA constant definitions has been introduced to build-specific configurations for conda-forge. It is currently necessary to handle some vendored dependencies using submodules (such as lmdb and lmdbxx which we do not want to maintain forks of) by modifying includes paths.

Moreover, the ARCTICDB_USING_STATIC_XXHASH is also used to be able to compile xxhash via its distributions on conda-forge.

Using logic based on constant definitions to adapt build-specific configuration is bad practice. Hence, we must adapt the build configuration to use CMake commands instead and remove the logic using ARCTICDB_USING_CONDA and ARCTICDB_USING_STATIC_XXHASH in the C++ headers. This might involve stopping using submodules and contributing to some dependencies.