Involves prefixing with $(srcdir) to some of the build rule variables. For
the builddir we create Makefiles which simply include the srcdir Makefile.
Add list make rule to get list of Makefiles to generate (find will get it
wrong for builddirs nested in srcdir).
There are still a few minor issues like the file paths echoed during make
install ...
Makes it more obvious which files are actually installed. MKINSTALLDIRS
still spurs out a lot of 'mkdir ...', which probably should also be
silenced when V=0.