easy way to switch from bundled libs to system

We don't like use bundled libraries in Fedora[0].
In this patch-set I want to improve easy switch.

General idea:
If system lib found - use it. If not found - use bundled.

In patch #3 I will apply patch if system bullet has version >= 2.81, because
there was changes and STK can't compile.

In the future I will update bundled bullet to 2.81, merge that patch and drop it.

[0]https://fedoraproject.org/wiki/Packaging:No_Bundled_Libraries

I've sent this patches to devel lists, but for sure sending pull request here..
http://sourceforge.net/mailarchive/message.php?msg_id=31856369
http://sourceforge.net/mailarchive/message.php?msg_id=31856370
http://sourceforge.net/mailarchive/message.php?msg_id=31856371
http://sourceforge.net/mailarchive/message.php?msg_id=31856372