[libpng12] Quiet unused variable warnings from clang by porting PNG_UNUSED()

from libpng-1.4.6.
[libpng12] Imported from libpng-1.2.51rc03.tar
[libpng12] Removed a stray semicolon after PNG_UNUSED(params) in pngread.c
[libpng12] Update "last changed" info in source files.
[libpng12] Added "-D_SOLO" to contrib/pngminim makefiles (for zlib-1.2.6+)
[libpng12] Imported from libpng-1.2.51rc04.tar
[libpng12] Added an #ifdef PNG_FIXED_POINT_SUPPORTED/#endif in pngset.c
[libpng12] Imported from libpng-1.2.51.tar
[libpng12] Bump version to 1.2.52beta01