and contrib/pngminim/preader/pnglibconf.dfa

depending on PNG_16BIT_SUPPORTED

and fixed a spelling error

in pngpriv.h in case the user forgot to define them in their pngusr.h.  This
means it is safe for people not building a DLL to omit these defines from pngusr.h.

Also corrected zlib.vcxproj into which Visual Studio had introduced
what it calls an "authoring error".  The change to make pnglibconf.h
simply copies the file; in the future it may actually generate the
file from scripts/pnglibconf.dfa as the other build systems do.

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)