Update of the build system and tools

- Reimplemented the logic for indicating the requirements and dependencies of examples.
- Updated the examples to indicate which texture or other resources they are using.
- Updated the build system to make examples dependent on textures which they use.
- Updated gl.hpp, added #ifndef OGLPLUS_NO_SITE_CONFIG around the include
  of the generated site_config.hpp header.
Update of the build system

- Added a check if threads are working.
- Updated requirements for examples requiring threads.
Removed the devel directory

- Also updated the _git/info/exclude file.
Update of the build system

- Added correct handling of return codes from cmake
  in the configure.py script.
- Added dependencies and requirements for example harnesses.
- Reimplemented the code checking for harness requirements
  and adding harness dependencies to be more scalable.
- Updated the requirements of some of the examples.
Merge branch 'develop' into feature-oalplus