src/common: use single Makefile for parallel builds

Use a single Makefile in 'src/common' as it contains multiple
subdirectories with a small number of objects to compile. This allows
faster parallel builds since parallelism in automake is applied per
Makefile.

There is anectodal evidence of a 25 seconds improvement to the build
process on a 36 core machine.

Change-Id: If2ce266050e345d58b00bf65b574ccf5168f28f1
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>