compiling postgres on windows - how to deal with unmatched file extension?

I'm trying to build postgresql from source code on windows. The build ran into a bunch of errors like 
'src/backend/bootstrap/bootparse.c': No such file or directory
'src/backend/parser/gram.c': No such file or directory

These .c files are referenced in postgres.vcxproj. They actual exist but their extension is either .l or .y instead of .c. What am I supposed to do so the build process will find these files with correct extension? I don't think I need to modify vcxproj files to make it work.
I checked out source code from the latest master version on github. I was expecting the build would go smoothly since I didn't modify anything.
If this is not the right group to post this kind of question, please let me know the right one.
Thanks,Andy