Question about common_files

[Lars Hecking]

 I have a software package that includes another package in a subdirectory:

 package_a/
 ...
 package_a/package_b/
 ...

 In order to build a standalone version of package_b, I'm running the following
 commands to make sure that no auto* generated files reference the parent dir:

 cd package_a/package_b/
 aclocal
 automake
 autoconf
 ./configure
 make dist

 So far so good. However, I notice that after make dist, the files
 install-sh missing mkinstalldirs have been copied from the parent
 directory:

 Makefile.in:
 DIST_COMMON = ... ../install-sh ../missing ../mkinstalldirs

 Question: how does automake know that these files are in the parent directory?
 Does it always check the parent directory for such files?

 Not that I'm complaining about this, just want to know what's going on :)