That way the compiler will report warnings against the original source, and not the pre-processed file.
commit 50dd27460503311919f41c0c2ad58f150875fd3a Author: Andrew Cagney <cagney@redhat.com> Date: Mon Apr 14 14:20:01 2008 -0400 frysk-common/ChangeLog 2008-04-14 Andrew Cagney <cagney@redhat.com> * Makefile.rules (SUBST): For .c and .cxx, add a #line directive pointing to the original file.