used cmake_modules to find TinyXML

This commit follows the pattern from the ROS core developers to
use cmake_modules to find TinyXML. E.g., This pattern is found in
the pluginlib package, commit 2e0860ca [1], and this commit here
bluntly copies the steps of that commit.

This commit is needed to find tinyxml when cross-compiling
the ar_track_alvar package with the OpenEmbedded/Yocto build
environment and the ROS layer for OpenEmbedded [2].

[1] https://github.com/ros/pluginlib/commit/2e0860ca42138f837b2e34921a207d40a095d996
[2] https://github.com/bmwcarit/meta-ros

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@oss.bmw-carit.de>
Merge pull request #24 from bulwahn/hydro-devel

used cmake_modules to find TinyXML