Use Stats.TargetStats record instead of an ugly tuple
CompatMakefile: Add .PHONY declarations
CompatMakefile: Depend on all existing accessed non-dir files

(cannot depend on the inexisting files or directories...)

This adds all inputs to build stats (whether or not compat makefile is
needed) which slightly slows down the build (~5% of the empty build
time).

This makes compat-makefile builds more correct but still not correct.