Whitespace
Get data directory from "data" file/dir in exe dir

If "data" is a file, each line specifies a data directory to add.
Non-absolute paths are resolved relative to the exe directory.

This has two uses:
- The data file is used to tell the arx binary in the build dir where
  to load the data files from the source tree without installing them.
- In (portable) installs the data files can be positioned relative
  to the arx binary without needing to be in the same directory

Paths found in this way are added directly before the exe dir itself.

This is all done for both ${arx_PATH} and the actual exe dir.
arx-install-data: Add support for "data" subdir
Document "data" subdir/file in --list-dirs
CMake: Use file to point to data in source tree

This replaces copying/symlinking the data/core contents into the build
directory.
Ignore ARB_texture_non_power_of_two with old cards

Fixes: bug #539
Disable mipmaps when emulating NPOT textures
Prevent non-clamp wrap modes with NPOT textures
Implement clamp to edge for emulated NPOT textures

This is done by filling the extended portion of the textures with the
pixel values from the edges.
Use GLEW's GLAPIENTRY if GLEW is new enough