Also apply minor updates to makefile.gcc and makefile.msys

The branch 'libpng16' is for tracking libpng version 1.6.x.
The branch 'master' is for tracking the latest stable libpng version.

These branches will diverge again at the next major libpng upgrade.

The new libpng license comprises the terms and conditions from the zlib
license, and the disclaimer from the Boost license.

The legacy libpng license, used until libpng-1.6.35, is appended to the
new license, following the precedent established in the Python Software
Foundation License version 2.

From now on, the list of contributing authors shall be maintained in a
separate AUTHORS file.

The lists of previous contributing authors, mentioned in the legacy
libpng license and considered to be an integral part of that license,
are kept intact, with no further updates.

(Contributed by Kyle Bentley)

Although CMake version 3.0.2 was previously required, the policies
CMP0053 and CMP0054, introduced only in CMake version 3.1 and used
in this CMakeLists file, can have surprising effects under an older
CMake version.

Simplify the CMakeLists file by setting the global policy version
to 3.1, and remove the specific policy settings.

As an added bonus, remove the vestigial workarounds for all ancient
CMake versions.

Only use libm on Unix and Unix-like systems, except for Apple (where
it's unnecessary), and BeOS and Haiku (where it's unavailable).

Avoid searching for libm at explicit locations. They may be incorrect,
especially for cross-platform builds.

Also apply style and formatting fixes

ARM-specific optimization processes 8 or 4 pixels at once.
Improves performance by around 10-22% on a recent ARM Chromebook.

(Contributed by Zhijie Liang)

Found via `codespell` and `grep`

As per the const correctness rules, top-level const-ness of data
in automatic scopes does not propagate outside of these scopes
(unlike const-ness at lower levels, such as pointers to const data).

Previously, const was used liberally, but inconsistently across the
libpng codebase. Using const wherever applicable is not incorrect.
However, _consistent_ use of const is difficult to maintain in such
conditions.

In conclusion, we shall continue to use const only where doing so is
strictly necessary:

1. If a function guarantees that it will not modify an argument
   passed by pointer, the corresponding function parameter should be
   a pointer-to-const (const T *).

2. Static data should not be modified, therefore it should be const.

Reference:
Google C++ Style Guide
https://google.github.io/styleguide/cppguide.html#Use_of_const

In v1.6.0, compiler support for const became a requirement.
It should be used consistently. To maintain backwards compatibility,
PNG_CONST is still maintained in deprecated form.

(Contributed by Theo Buehler)

The compatibility workaround png_memset() has been removed long ago.
The availability of memset() is a platform requirement.

Pre-ANSI function prototypes are no longer supported.

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.