All (but one) of these links point to local files created when you install WinAVR. They probably will not work if you install a different (than 200806) or do not install in the default directory.

WinAvr consists of two parts: the GNU C Compiler (compiled to run on Windows rather than Linux) and the standard C library, libgcc, coded specifically for the AVR processor family.

The GNU C Compiler comes from the Free Software Foundation started by Richard Stallman.  It supports most large processors (e.g. x86, PowerPC, ARM etc.) and several microcontrollers such as the AVR.  It is the standard compiler used in Linux.  It is used widely in the embedded indusrty because of its high quality continuous improvement, the large number of processors supported, its support for Linux, and its availability at zero cost.  There is a lot to know about the tool chain beyond the C language.  Whataver you learn about the GNU tool chain, you can likely use professionally in embedded programming.

The library part of WinAVR is not from GNU or the Free Software foundation, but also is free of cost, freely modifiable and maintained by volunteers.