Set SDK paths in ENV variables

[walkero-gr]

This Pull Request is about what I requested at #19

By applying this pull request every developer:

1. can keep the original AmigaOS 4 SDK files, without doing any changes
2. can keep the needed SDK files for compiling Odyssey in separate folder
3. can change the development environment the way he wants
4. can be more flexible to setup his system
5. can build a different system faster

What you need to do to make Odyssey compile again is either:

• create two environment variables at you system pointing to the paths where the SDK files exist, which are ODYSSEY_INC and AOS4_SDK_INC or
• change the variables CMAKE_EXTRA_SDK_PATH and CMAKE_AOS4_SDK_PATH in amigaos4.cmake file and hardcode your paths

ODYSSEY_INC and CMAKE_EXTRA_SDK_PATH should point to odyssey-r155188-1.23_SDK/SDK folder
AOS4_SDK_INC and CMAKE_AOS4_SDK_PATH should point to the folder where the AmigaOS 4 SDK include files are extracted. There should be at least the folders clib2, include and newlib, as they come with the latest AmigaOS 4 SDK 53.30.

Take this Pull Request as a proposal. Feel free to ignore it if you don't find it useful.