build: Add the CMake build option PNG_LIBCONF_HEADER

Running the pnglibconf scripts (script/*.awk) is not always possible.
An AWK interpreter is not always guaranteed to be available; and even
if it is, there are limitations when making cross-platform libpng
builds, especially when the differences between the host platform and
the target platform are significant. For example:
 * Building for the Windows (MinGW) target on a Unix development host;
 * Building for the iOS target on a macOS development host;
 * Building for the Android target on any development host.

In such scenarios, a preconfigured (i.e. prebuilt) pnglibconf.h file,
either taken from the libpng source tree or provided by the user who
wants to make a custom libpng build, becomes a necessity.

In this commit we introduce the build option `PNG_LIBCONF_HEADER` in
order to address this specific use case.

We also specify a version range (3.14...4.0) for the minimum required
CMake program, to future-proof the CMake build for just a little bit
longer.

Signed-off-by: Cosmin Truta <ctruta@gmail.com>

scripts/cmake/PNGCheckLibconf.cmake

@@ -0,0 +1,45 @@
+# PNGConfig.cmake
+# Utility functions for configuring and building libpng
+
+# Copyright (c) 2025 Cosmin Truta
+#
+# 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
+
+# Check libconf file (pnglibconf.h.* or *.dfa):
+# png_check_libconf([HEADER <file>] [DFA_XTRA <file>])
+function(png_check_libconf)
+  set(options)
+  set(oneValueArgs HEADER DFA_XTRA)
+  set(multiValueArgs)
+
+  cmake_parse_arguments(_CHK "${options}" "${oneValueArgs}" "${multiValueArgs}" ${ARGN})
+
+  if(_CHK_HEADER AND _CHK_DFA_XTRA)
+    message(FATAL_ERROR "png_check_libconf: Mutually-exclusive arguments: HEADER and DFA_XTRA")
+  endif()
+
+  if(_CHK_HEADER)
+    if(EXISTS "${_CHK_HEADER}")
+      if("x${_CHK_HEADER}" STREQUAL "x${PNG_LIBCONF_HEADER_PREBUILT}")
+        message(STATUS "Using standard libconf header: ${_CHK_HEADER}")
+      else()
+        message(STATUS "Using custom libconf header: ${_CHK_HEADER}")
+      endif()
+    else()
+      message(SEND_ERROR "Could not find libconf header: ${_CHK_HEADER}")
+    endif()
+  else()
+    if("x${_CHK_DFA_XTRA}" STREQUAL "x")
+      message(STATUS "Creating standard configuration")
+    elseif(EXISTS "${_CHK_DFA_XTRA}")
+      message(STATUS "Creating custom configuration with DFA_XTRA file: ${_CHK_DFA_XTRA}")
+    else()
+      message(SEND_ERROR "Could not find DFA_XTRA file: ${_CHK_DFA_XTRA}")
+    endif()
+  endif()
+endfunction()