build: Move portions from the main CMake file to separate modules

In preparation for the upcoming changes in the build system, any parts
of the CMake file that are unlikely to be affected should be moved out
of the way. This should facilitate an easier resync between the branch
'libpng16' and its successor(s).

Specifically:
 * Move the functions `generate_chk`, `generate_out`, `generate_source`
   and `generate_copy` to scripts/cmake/PNGGenConfig.cmake.
 * Move the function `png_add_test` to scripts/cmake/PNGTest.cmake.
 * Leave the function `create_symlink` in place, but add a TODO note.
   As we raised the minimum required CMake version to 3.14, we should
   now be able to use CMake's built-in function instead.

scripts/cmake/PNGTest.cmake

@@ -0,0 +1,42 @@
+# PNGTest.cmake
+# Utility functions for testing libpng
+
+# Copyright (c) 2018-2025 Cosmin Truta
+# Copyright (c) 2016-2018 Glenn Randers-Pehrson
+# Written by Roger Leigh, 2016
+#
+# 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
+
+# Add a custom target to run a test:
+# png_add_test(NAME <test> COMMAND <command> [OPTIONS <options>...] [FILES <files>...])
+function(png_add_test)
+  set(options)
+  set(oneValueArgs NAME COMMAND)
+  set(multiValueArgs OPTIONS FILES)
+  cmake_parse_arguments(_PAT "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})
+  if(NOT _PAT_NAME)
+    message(FATAL_ERROR "png_add_test: Missing NAME argument")
+  endif()
+  if(NOT _PAT_COMMAND)
+    message(FATAL_ERROR "png_add_test: Missing COMMAND argument")
+  endif()
+
+  # Initialize the global variables used by the "${_PAT_NAME}.cmake" script.
+  set(TEST_OPTIONS "${_PAT_OPTIONS}")
+  set(TEST_FILES "${_PAT_FILES}")
+
+  # Generate and run the "${_PAT_NAME}.cmake" script.
+  configure_file("${CMAKE_CURRENT_SOURCE_DIR}/scripts/cmake/test.cmake.in"
+                 "${CMAKE_CURRENT_BINARY_DIR}/tests/${_PAT_NAME}.cmake"
+                 @ONLY)
+  add_test(NAME "${_PAT_NAME}"
+           COMMAND "${CMAKE_COMMAND}"
+                   "-DLIBPNG=$<TARGET_FILE:png_shared>"
+                   "-DTEST_COMMAND=$<TARGET_FILE:${_PAT_COMMAND}>"
+                   -P "${CMAKE_CURRENT_BINARY_DIR}/tests/${_PAT_NAME}.cmake")
+endfunction()