The Try/Catch blocks were checking that the context had not changed -
unnecessary and cexcept cannot handle separate (potentially interleaved)
contexts anyway.  Changed the code to make the Catch re-instate the context -
this should help the compiler because the context is the png_store and
minimising the need for the compiler to avoid optimizing it away is good.

 now that cexcept.h is in place.

installing libpng in /usr/lib64 (Funda Wang).  Also ported other recent
changes from libpng-1.4.4beta07:
  Revised CMakeLists.txt to put the man pages in share/man/man* not man/man*
  Revised CMakeLists.txt to make symlinks instead of copies when installing.
  Changed PNG_LIB_NAME from pngNN to libpngNN in CMakeLists.txt (Philip Lowman)

in an earlier update.  Fixed to declare the auto variables at the head.

and shortening problems on assignment but also a few difficult to optimize
(for GCC) loops (John Bowler).

Also added scripts/chkfmt to validate the format of all the files that can
reasonably be validated (it is suggested to run "make distclean" before
checking, because some machine generated files have long lines.)
Reformatted the CHANGES file to be more consistent throughout.

literal pointers declare them (John Bowler).
Many APIs did not change their arguments but were not declared using
PNG_CONST.  This change corrects this.  In a few cases APIs that return
constant string literal pointers have also been changed to declare this.
Unlike the argument change this may require app changes; however the
results could never be written to (the app would crash on some platforms
where strings are not writable), so this seems advisable.

Revised contrib/gregbook to work under cygwin/mingw.

they are no longer used.

  The changes include support for function attributes in VC in addition to
    those already present in GCC - necessary because without these some
    warnings are unavoidable.  Fixes include signed/unsigned fixes in
    pngvalid and checks with gcc -Wall -Wextra -Wunused.
  VC requires function attributes on function definitions as well as
    declarations, PNG_FUNCTION has been added to enable this and the
    relevant function definitions changed.