[libpng16] Removed 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.
[libpng16] Protect symbol prefixing against CC/CPPFLAGS/CFLAGS useage.

Previous iOS/Xcode fixes for the ARM NEON optimizations moved the test
on __ARM_NEON__ from configure time to compile time.  This breaks symbol
prefixing because the definition of the special png_init_filter_functions
call was hidden at configure time if the relevant compiler arguments are
passed in CFLAGS as opposed to CC.  This change attempts to avoid all
the confusion that would result by declaring the init function even when
it is not used, so that it will always get prefixed.