Add files generated by CMake to gitignore.
Add basic CMake build setup
Enable compiler warnings through compile options
Add a section about building the library
Add link to cmake website.
Merge pull request #39 from neithernut/add-build-env

Add build env
Added avl.{c,h} files with first definition of the interface
Added implementation of avl_alloc(), which is just an alias for convenience
Added comperator func ptr arg to avl_add()

And adapted avl_alloc() appropriately.
Added infrastructure in avl_add() function
Fixup of the parameters of the find() and del() functions
Added helper function for finding elements
Added infrastructure implementation for avl_del() function
Added more comment to the helper function
Added implementation of the avl_find() function
Fixup for the initial import (argument types)
Added basic infrastructure implementation of the avl_unlink() function
Fix: We should name the data variable "data", not "d"

Suggested-by: Marcel Mueller <com2040@gmail.com>
Added documentation for the AVL tree node data type

Suggested-by: Marcel Mueller <com2040@gmail.com>
Added simple implementation for avl_destroy()