Gitignore and autogen.sh

[SiggyF]

This change updates the .gitignore file so that a git status shows only the changed files and ignores all the autotools/make check/make results. It also adds the (BSD 3-Clause Licensed) autogen.sh script which generates more understandable error messages and warns about issues (glibtoolize vs libtoolize on OSX for example).

[SiggyF]

If you prefer to only ignore "out of source" builds in the "build" directory I created a simplified version in https://github.com/SiggyF/netcdf-c/blob/gitignore/.gitignore . This ignores everything in build and some autotools stuff.

[WardF]

After reviewing this pull request, we've decided to close this pull request. Ignoring the autotools-generated files (not normally distributed with the development code, but distributed with the releases/tags) would require changes to our workflow for releases. Given our lack of manpower (currently one full time developer and two half-time developers), we can't spare the resources for this. Similarly, we're going to investigate the autogen.sh script, but our lack of resources makes changing out this chunk of our workflow burdensome. It's not that we're not interested, but we have to choose how to best invest our time; that said, thank you very much for generating the pull request, we appreciate your contribution, even if we can't immediately make use of it!