and png_convert_to_rfc1123(). (Frank Busse).

Some compilers fault 'extern const' data declarations (because the data is
not initialized); this turns on const-ness only for compilers where
this is known to work.

    using g++. The compiler imposes C++ rules on the C source; thus it
    is desireable to make the source work with either C or C++ rules
    without throwing away useful error information.  This change adds
    png_voidcast to allow C semantic (void*) cases or the corresponding
    C++ static_cast operation, as appropriate.

in pngpriv.h and png.c

The slightly modified tables reduce the number of 16-bit values that
convert to an off-by-one 8-bit value.  The "makesRGB.c" code that was used
to generate the tables is now in a contrib/sRGBtables sub-directory.

on a variety of images, limited documentation (in the header file.)

These fixes attend to most of the errors revealed in pngvalid, however doing
the gamma work twice results in inaccuracies that can't be easily fixed.
There is now a warning in the code if this is going to happen.