PNG_GAMMA_MAC_18 as parameters for png_set_gamma().

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.

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.

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.

(Andrew Hundt)

descriptions

with the right zlib headers.

Added information in the documentation about problems with and fixes for
the bad CRC and bad iTXt chunk situations.

which requires both the correct libpng and the correct zlib to function
correctly.

Fixed whitespace handling
Added a call to png_set_packing()
Initialize dimension values so if sscanf fails at least we have known
   invalid values.

    only works with libpng versions that can be made to reliably fail when
    the deflate data contains an out-of-window reference.  This means only
    1.6 and later.
  Fixed gnu issues: g++ needs a static_cast, gcc 4.4.7 has a broken warning
    message which it is easier to work round than ignore.

For inflate, reverted previous fixes.
Added fixitxt and pngdeflate to the built programs and removed warnings
from the source code and timepng that are revealed as a result.  Fixed
fixitxt when the chunk length is more than 65535 (untested, no test case).

Ignore "#" delimited comments in input file to pnm2png.c.

trusting the CMF bytes in the PNG datastream.