Fix for universal binaries on macOS

Fix the issue of a
    clang: error: no such file or directory: 'x86_64'
error on building universal binaries on macOS.

It is also necessary to specify:
    -DCMAKE_C_FLAGS="-DPNG_ARM_NEON_OPT=0"

Example CMake command:
    cmake -S libpng \
          -B build_libpng \
          -DCMAKE_OSX_ARCHITECTURES="arm64;x86_64" \
          -DCMAKE_C_FLAGS="-DPNG_ARM_NEON_OPT=0"

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

Author: Jon Creighton

CMakeLists.txt

@@ -1,6 +1,6 @@
 # CMakeLists.txt
 
-# Copyright (c) 2018-2022 Cosmin Truta
+# Copyright (c) 2018-2023 Cosmin Truta
 # Copyright (c) 2007,2009-2018 Glenn Randers-Pehrson
 # Written by Christian Ehrlicher, 2007
 # Revised by Roger Lowman, 2009-2010
@@ -20,6 +20,7 @@
 # Revised by Gleb Mazovetskiy, 2021
 # Revised by Christopher Sean Morrison, 2022
 # Revised by Martin Storsjo, 2022
+# Revised by Jon Creighton, 2023
 
 # This code is released under the libpng license.
 # For conditions of distribution and use, see the disclaimer

scripts/genout.cmake.in

@@ -24,10 +24,10 @@ set(ZLIBINCDIR "@ZLIB_INCLUDE_DIRS@")
 
 set(PLATFORM_C_FLAGS)
 if(APPLE)
-  set(CMAKE_OSX_ARCHITECTURES "@CMAKE_OSX_ARCHITECTURES@")
+  set(CMAKE_OSX_INTERNAL_ARCHITECTURES "@CMAKE_OSX_INTERNAL_ARCHITECTURES@")
   set(CMAKE_OSX_SYSROOT "@CMAKE_OSX_SYSROOT@")
-  if(CMAKE_OSX_ARCHITECTURES)
-    set(PLATFORM_C_FLAGS ${PLATFORM_C_FLAGS} -arch ${CMAKE_OSX_ARCHITECTURES})
+  if(CMAKE_OSX_INTERNAL_ARCHITECTURES)
+    set(PLATFORM_C_FLAGS ${PLATFORM_C_FLAGS} -arch ${CMAKE_OSX_INTERNAL_ARCHITECTURES})
   endif()
   if(CMAKE_OSX_SYSROOT)
     set(PLATFORM_C_FLAGS ${PLATFORM_C_FLAGS} -isysroot ${CMAKE_OSX_SYSROOT})