[libpng15] Revised example.c to illustrate use of PNG_DEFAULT_sRGB and

PNG_GAMMA_MAC_18 as parameters for png_set_gamma().  These have been available
since libpng-1.5.4.
[libpng15] Imported from libpng-1.5.17rc03.tar
[libpng15] Imported from libpng-1.5.17.tar
[libpng15] Bump version to 1.5.18beta01
[libpng15] Ported recent "/*SAFE*/" comments from libpng-1.6.3