Add gitattributes and gitignore files

[rleigh-codelibre]

• gitignore ignores all generated files for Autotools, CMake and build systems make, ninja, nmake and msbuild
• gitattributes defines line ending conventions so that cross-platform development doesn't arbitrarily change the line ending style under source control, resulting in whole-file diffs and conflicts; this requires a one-time renormalisation of the line endings stored internally by git (done here). The line ending style on checkout is configurable; defaults to LF on Unix, CRLF on Windows

You can view the whitespace-only diff to see the changes without the line-ending renormalisation.