of parallelism during 'make' the use of the same temporary file names such
as 'dfn*' can result in a race where a temporary file from one arm of the
build is deleted or overwritten in another arm.  This changes the
temporary files for suffix rules to always use $* and ensures that the
non-suffix rules use unique file names.

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.