[libpng16] style: Add a cmake-format config file and satisfy cmake-lint

Introduce CMake file linting, based on the cmakelang project by
Josh Bialkowski (@cheshirekow).

Fix various style issues in the CMake files:
 * Add the missing copyright header to PNGConfig.cmake.
 * Resolve a "missing docstring" warning raised by cmake-lint.
 * Rewrite all docstrings in the style of CMake's own documentation.
 * Fix whitespace inconsistencies.

References:
 * https://pypi.org/project/cmakelang
 * https://github.com/cheshirekow/cmake_format

This is a cherry-pick of commit c317fe31114d92d05d44bf98c9b559edf4656b9c
from branch 'libpng18'.

scripts/cmake/PNGConfig.cmake

@@ -1,15 +1,28 @@
-include(CMakeFindDependencyMacro)
-
-find_dependency(ZLIB REQUIRED)
-
-include("${CMAKE_CURRENT_LIST_DIR}/PNGTargets.cmake")
-
-if(NOT TARGET PNG::PNG)
-  if(TARGET PNG::png_shared)
-    add_library(PNG::PNG INTERFACE IMPORTED)
-    target_link_libraries(PNG::PNG INTERFACE PNG::png_shared)
-  elseif(TARGET PNG::png_static)
-    add_library(PNG::PNG INTERFACE IMPORTED)
-    target_link_libraries(PNG::PNG INTERFACE PNG::png_static)
-  endif()
-endif()
+# PNGConfig.cmake
+# CMake config file compatible with the FindPNG module.
+
+# Copyright (c) 2024 Cosmin Truta
+# Written by Benjamin Buch, 2024
+#
+# Use, modification and distribution are subject to
+# the same licensing terms and conditions as libpng.
+# Please see the copyright notice in png.h or visit
+# http://libpng.org/pub/png/src/libpng-LICENSE.txt
+#
+# SPDX-License-Identifier: libpng-2.0
+
+include(CMakeFindDependencyMacro)
+
+find_dependency(ZLIB REQUIRED)
+
+include("${CMAKE_CURRENT_LIST_DIR}/PNGTargets.cmake")
+
+if(NOT TARGET PNG::PNG)
+  if(TARGET PNG::png_shared)
+    add_library(PNG::PNG INTERFACE IMPORTED)
+    target_link_libraries(PNG::PNG INTERFACE PNG::png_shared)
+  elseif(TARGET PNG::png_static)
+    add_library(PNG::PNG INTERFACE IMPORTED)
+    target_link_libraries(PNG::PNG INTERFACE PNG::png_static)
+  endif()
+endif()