Fixed an issue with $LIBS in the configure script

- It is now possible to pass custom LIBS values to ./configure by
  specifying them on the command line

Signed-off-by: Haru <haru@dotalux.com>
Added support for other libmysqlclient variants

- This allows Hercules to correctly detect MariaDB and other drop-in
  replacements in distributions that rename their libmysqlclient (i.e.
  Debian / Ubuntu renaming it to libmariadbclient).
- Closes issue #368
- Due to a bug in Ubuntu 14.04 LTS (incorrect output by the
  mariadb-provided version of `mysql_config --libs` and missing
  dependencies of libmariadbclient-dev), users of said distribution will
  have to manually specify ./configure LIBS="-lssl -lcrypto" as well as
  manually install the libssl-dev package, if they wish to use
  libmariadbclient-dev, as it will be unable to link otherwise (note:
  this issue isn't Hercules-specific)
- Special thanks to Andrei Karas

Signed-off-by: Haru <haru@dotalux.com>