Allow MYSQL_LIB to be specified by ENV variable

[MichalMMac]

When I was compiling Icinga2 on macOS I run into minor issue when I had MariaDB client libs located on path (/opt/local/lib/mariadb/mysql/) FindMySQL.cmake was unable to find. It was not possible to specify MYSQL_LIB entirely by environment variable (like MYSQL_INCLUDE_DIR).

This PR allows to specify MYSQL_LIB path entirely from MYSQL_DIR environment variable.

Not sure if MYSQL_DIR is best var name for this case. It might be better to change it to something else ( maybe MYSQL_LIB_DIR?).

[gunnarbeutner]

LGTM. 👍

[gunnarbeutner]

I'm OK with MYSQL_DIR. I don't think we're explicitly setting it anywhere atm but I'd rather not rename it - mainly to keep things in line with the upstream version of that file.

Merged. Thank you. :)