How to install or update mysql and postgres gems

Good day!
I have problems with installing and updating mysql and postgres gems.
It seems like RubyGems can’t find my mysql and postgres installations
(they are in /usr/local/mysql and /usr/local/pgsql as by default). What
flag I should supply to gem to install and update successfully.
Is there any way to make gems permanently know about these locations so
that I will be able to use gem update to update all?
I use RubyGems 0.9.1.

Your sincerely,
Damian/Three-eyed Fish