Makefile.am: added populate-makefiles target

This patch adds a new target to the root-level Makefile "populate-makefiles"
which copies a skeleton Makefile to all subdirectories of the build
tree. This allows subdirectory builds even though we use only a single
Makefile in the root directory.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>