more png

Got some undeclared identifiers:

c:\cygwin_home\dave\workarea\png\libpng-1.2.18\pngvcrd.c(315) : error C2065: 'PNG_ASM_FLAG_MMX_READ_COMBINE_ROW' : undeclared identifier
c:\cygwin_home\dave\workarea\png\libpng-1.2.18\pngvcrd.c(1219) : error C2065: 'PNG_ASM_FLAG_MMX_READ_INTERLACE' : undeclared identifier
c:\cygwin_home\dave\workarea\png\libpng-1.2.18\pngvcrd.c(3700) : error C2065: 'PNG_ASM_FLAG_MMX_READ_FILTER_SUB' : undeclared identifier
c:\cygwin_home\dave\workarea\png\libpng-1.2.18\pngvcrd.c(3703) : error C2065: 'PNG_ASM_FLAG_MMX_READ_FILTER_UP' : undeclared identifier
c:\cygwin_home\dave\workarea\png\libpng-1.2.18\pngvcrd.c(3706) : error C2065: 'PNG_ASM_FLAG_MMX_READ_FILTER_AVG' : undeclared identifier
c:\cygwin_home\dave\workarea\png\libpng-1.2.18\pngvcrd.c(3709) : error C2065: 'PNG_ASM_FLAG_MMX_READ_FILTER_PAETH' : undeclared identifier

These are resolved by defining PNG_MMX_CODE_SUPPORTED. I don't know if this is the best way (or even a good way) to fix this.