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.

correction.

in pngpriv.h

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.

instead of ANNOUNCE

adding alpha and reducing to 8-bit

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

and pnglibconf.mak

to handle the new PNG_JOIN macro.

the sBIT fields in the test pixel as 0, which resulted in a floating
point division by zero which was irrelevant but causes systems where
FP exceptions cause a crash. Added code to pngvalid to turn on FP
exceptions if the appropriate glibc support is there to ensure this is
tested in the future.

casts. The compression_type parameter is always assigned to, so must
be non-NULL. The cast of the profile length potentially truncated the
value unnecessarily on a 16-bit int system, so the cast of the (byte)
compression type to (int) is specified by ANSI-C anyway.