create_symlink breaks the MSYS generator in the Windows CMake.

Delete the files that are unused, redundant, or fundamentally broken:
def.c, makefile.bor, makefile.knr, makefile.msc, makefile.solaris-x86,
makefile.tc3

As of July 2018, this statement is no longer necessary.

Also reformat the license text.

This information is maintained by the version control system.

(Contributed by github.com/irwir)

(Bug report by Thuan Pham, SourceForge issue #278)

In v1.6.0, size_t became a required type. It should be used
consistently. To maintain backwards compatibility, png_size_t
is still maintained in deprecated form.

found via `codespell-q 3`
please review

Always define PNG_INTEL_SSE_OPT and PNG_INTEL_SSE_IMPLEMENTATION before
comparing them with 0.

Some are user facing. Some are in actual code. Most are in source comments. Also, please double check the changes in contrib/tools/pngfix.c