[libpng16] Added set(CMAKE_CONFIGURATION_TYPES ...) to CMakeLists.txt

(Andrew Hundt)
[libpng16] Deleted set(CMAKE_BUILD_TYPE) block from CMakeLists.txt
[libpng16] Fix compiler warning in png-fix-itxt.c
[libpng16] Updated README (removed CompuServ reference, added simplesystems.org)
[libpng16] Enclose the prototypes for the simplified write API in