GNUmakefile: disable parallelism only for multiple, recursive targets

GNUmakefile: disable parallelism only for multiple, recursive targets

* top/GNUmakefile (ALL_RECURSIVE_TARGETS): New macro; allow user
additions in the Makefile.
(AM_RECURSIVE_TARGETS): New macro, override only if not provided
by Automake.
(.NOTPARALLEL): Only disable parallel builds if multiple targets
are listed on the command line and at least one of them is
listed in $(ALL_RECURSIVE_TARGETS).

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>