[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
[libpng16] Make ARM NEON support work at compile time (not just configure time).

This moves the test on __ARM_NEON__ into pngconf.h to avoid issues when
using a compiler that compiles for multiple architectures at one time.
[libng16] Neglected to check in pngconf.h with previous commit.
Fix NEON optimizations in some more environments
This removes PNG_FILTER_OPTIMIZATIONS and PNG_ARM_NEON_SUPPORTED from
pnglibconf.h, allowing more of the decisions to be made internally (pngpriv.h)
during the compile.  Without this symbol prefixing is broken under certain
circumstances on ARM platforms.  Now only the API parts of the optimizations
('check' vs 'api') are exposed in the public header files except that the new
setting PNG_ARM_NEON_OPT documents how libpng makes the decision about whether
or not to use the optimizations.

Signed-off-by: John Bowler <jbowler@acm.org>