Use MARIADB_BASE_VERSION instead of LIBMARIADB to detect Maria DB

It was discovered that MariaDB 10 didn't define LIBMARIADB leading
to compilation errors of MySQL DLZ modules on Debian stretch.

Use MARIADB_BASE_VERSION instead which is defined in all tested MariaDB
versions.
65 jobs for !6213 with 2813-build-named-with-dlz-is-broken-with-MariaDB-10 in 7 seconds (queued for 5 seconds)
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