Support for MPW Makefile ".y" extensions in addition to ".o"

[steventroughtonsmith]

I noticed a gotcha when porting some old MPW makefiles (https://github.com/steventroughtonsmith/SimpleText) over in that object files often come with a .y file extension; if you don't remap that to .o in your new makefile the build process will fail horribly at the linker stage.

Not specifically a 'bug' in mpw's case, but worth noting here at least.