package/libmdbx: new package (library/database).

package/libmdbx: new package (library/database).

This patch adds libmdbx v0.9.2:
 - libmdbx is one of the fastest compact embeddable key-value ACID database.
 - libmdbx has a specific set of properties and capabilities,
   focused on creating unique lightweight solutions.
 - libmdbx surpasses the legendary LMDB (Lightning Memory-Mapped Database)
   in terms of reliability, features and performance.
 - https://github.com/erthink/libmdbx

Signed-off-by: Leonid Yuriev <leo@yuriev.ru>
[yann.morin.1998@free.fr: split long lines]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>