[libpng16] Use pnglibconf.h.prebuilt when building for ANDROID with cmake

Github issue 162, by rcdaily).

CMakeLists.txt

@@ -262,7 +262,7 @@ find_program(AWK NAMES gawk awk)
 
 include_directories(${CMAKE_CURRENT_BINARY_DIR})
 
-if(NOT AWK)
+if(NOT AWK OR ANDROID)
   # 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)