What is the purpose of using Makefile_insert to pretend that everything is in
the src directory instead of using the SUBDIRS directive?
I'm intending to change the Automake build system to link the executables
from single object files instead of static libraries so that we may finally
put files in the proper directories (e.g. remove widgets/ and dialogs/ and
move those to ui/widget and ui/dialog/, as well as join helper/, util/ and
svg/), and I would like to know whether I should also sanitize general
Makefile usage while I'm at it.