and rebuilt Makefile.in and configure, etc., with autoconf-2.68

(clarifying that the "lang" and "lang_key" fields can be NULL pointers
or might not exist).

and png_convert_to_rfc1123(). (Frank Busse).

Bug report by Franke 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.

correction.