Parallel make output in $(shell ${MAKE} ...)

[Glenn Morris]

Hi,

I added a construct like the following to the Emacs Makefiles recently:

  texi_misc = $(shell ${MAKE} --no-print-directory -s -C doc/misc echo-sources)

  info/dir: ${texi_misc}
     ...do something...

Obviously that's simplified; the full thing can be seen at:
  https://git.savannah.gnu.org/cgit/emacs.git/tree/Makefile.in#n1011

The doc/misc/Makefile "echo-sources" target echoes a list of source
files to stdout.

It seems that texi_misc can end up containing parallel make output, eg
https://lists.gnu.org/r/emacs-devel/2021-03/msg00305.html

I found it surprising that $(shell ) could get polluted in this way.

Is there a way to avoid this problem, that is portable to GNU make >= 3.81?

Thanks.