fix: Group Targets requires GNU Make >= 4.3

Group Targets requires GNU Make >= 4.3 released in 2020, with older Make
release the syntax only results in a warning and most of the time it
will work fine since the rule is called and generates both files.
However it breaks the dependencies across files and makes the man pages
generation racy.

Use a less elegant workaround that is compatible with all Make
implementations.

Change-Id: I46a0488c6ff61082f641d8ee55ad2802c0bfa046
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>