s are built with the target's normal cflags, but they
    obviously don't get the |-include| flag. Instead, they need a -x flag that
    describes their language.
  + All o files in the target need to depend on the gch file, to make sure
    it's built before any o file is built.

This class helps with some of these tasks, but it needs help from the build
system for writing dependencies to the gch files, for writing build commands
for the gch files, and for figuring out the location of the gch files.
c