[libpng16] Removed one of the GCC-7.1.0 'strict-overflow' warnings that

result when integers appear on both sides of a compare.  Worked around the
  others by forcing the strict-overflow setting in the relevant functions to
  a level where they are not reported.
Changed "FALL THROUGH" comments to "FALLTHROUGH" because GCC doesn't like
  the space.
Worked around some C-style casts from (void*) because g++ 5.4.0 objects
  to them.
Increased the buffer size for 'sprint' to pass the gcc 7.1.0 'sprint
  overflow' check that is on by default with -Wall -Wextra.

contrib/libtests/pngunknown.c

@@ -1,8 +1,8 @@
 
 /* pngunknown.c - test the read side unknown chunk handling
  *
- * Last changed in libpng 1.6.26 [October 20, 2016]
- * Copyright (c) 2015,2016 Glenn Randers-Pehrson
+ * Last changed in libpng 1.6.31 [(PENDING RELEASE)]
+ * Copyright (c) 2015,2017 Glenn Randers-Pehrson
  * Written by John Cunningham Bowler
  *
  * This code is released under the libpng license.
@@ -614,7 +614,7 @@ get_unknown(display *d, png_infop info_ptr, int after_IDAT)
                   ++(d->error_count);
                   break;
                }
-               /* FALL THROUGH (safe) */
+               /* FALLTHROUGH */ /* (safe) */
             case PNG_HANDLE_CHUNK_ALWAYS:
                break;
          }