Problems with MySQL Installation

I borrow this thread because I have the same problem but with sqlite3.

...
checking for sqlite3.h... no
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers.
...

I tried to fix it by downloading the source code for sqlite from
http://www.sqlite.org/ and installing it. Now i have the sqlite3.h
header file and tried again with

gem install sqlite3-ruby --
--with-sqlite3-dir=/my/sqlite/path/sqlite-3.6.17

The sqlite3.h file is in that directory but I get the same errors
again..

Do you really have sqlite3 installed? What happens when you type:
which sqlite3

i.e.: which sqlite3
/opt/local/bin/sqlite3

On my system I would then install the gem using:

gem install sqlite3-ruby -- --with-sqlite3-include=/opt/local/include
--with-sqlite3-lib=/opt/local/lib