cmake: Fix a build error on iOS

Avoid the awk-based configuration when cross-compiling from macOS
to iOS.

CMakeLists.txt

@@ -14,6 +14,7 @@
 # Revised by Cosmin Truta, 2018
 # Revised by Kyle Bentley, 2018
 # Revised by David Callu, 2020
+# Revised by Steve Robinson, 2020
 
 # This code is released under the libpng license.
 # For conditions of distribution and use, see the disclaimer
@@ -273,7 +274,7 @@ find_program(AWK NAMES gawk awk)
 
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
 
-if(NOT AWK OR ANDROID)
+if(NOT AWK OR ANDROID OR IOS)
   # No awk available to generate sources; use pre-built pnglibconf.h
   configure_file(${CMAKE_CURRENT_SOURCE_DIR}/scripts/pnglibconf.h.prebuilt
                  ${CMAKE_CURRENT_BINARY_DIR}/pnglibconf.h)
@@ -452,7 +453,7 @@ else()
     "${CMAKE_CURRENT_BINARY_DIR}/scripts/symbols.chk"
     "${CMAKE_CURRENT_BINARY_DIR}/scripts/symbols.out"
     "${CMAKE_CURRENT_BINARY_DIR}/scripts/vers.out")
-endif(NOT AWK OR ANDROID)
+endif(NOT AWK OR ANDROID OR IOS)
 
 # OUR SOURCES
 set(libpng_public_hdrs
@@ -466,7 +467,7 @@ set(libpng_private_hdrs
   pnginfo.h
   pngstruct.h
 )
-if(AWK AND NOT ANDROID)
+if(AWK AND NOT ANDROID AND NOT IOS)
   list(APPEND libpng_private_hdrs "${CMAKE_CURRENT_BINARY_DIR}/pngprefix.h")
 endif()
 set(libpng_sources