build: Move scripts/*.cmake.in to scripts/cmake/; add cmake/AUTHORS.md

From the libpng licensing point of view, the build projects, the build
scripts, the test scripts, the CI verification scripts, et cetera, have
not traditionally been part of libpng proper, although some of these,
including the CMake-based build, have been released under the libpng
license.

Considering how the CMake build grew as a result of many contributions
from many contributing authors over a long time, one may argue that it
almost became an individual piece of software in its own right.

Moving on, everything CMake-related shall be placed in the subdirectory
scripts/cmake/ (except, of course, the main CMakeLists.txt). Moreover,
contributing authors shall be acknowledged in scripts/cmake/AUTHORS.md.

Please see scripts/cmake/README.md for more information.
7 files changed