Updated the GL detection in the build system

- Added a file checking if all required functions (according to gl.xml) are available.
Merge branch 'release-0.41.0' into develop
Multiple changes

- Updated news feed source.
- Minor bug fix. Fixes #47.
Moved some functions into the implement/ dir.
Moved some functions to the implement/ dir.
Added a new debugging option to the build system

- Also updated the bash-completion script and manual for configure.
Multiple changes

- Minor updates to GL version detection.
- Added #ifs to sources to disable some features if a required GL version
  is not available.
Update of readme.
Update of readme.
Minor doc typo fixup.
Minor update of readme.
Added a standalone example
Removed one of the older oglplus examples
Updated GL version detection/reporting.
Minor changes related to configure bash-completion
Update of the configure script

- If LD_LIBRARY_PATH is set the configure automatically adds the listed
  directories to the cmake library search paths.
Fixed a problem in GLSLSource

- Fixed a problem with reading the source from istreams for which
  the size cannot be determined reliably.
Updated error handling in GLSLSource::FromStream
Minor update in GLSLSource
Added warning guards for visual studio in limited_value.hpp to suppress template warning 4396 on VS2010.