dist-docs: Make GNU make aware of sub-make.

When GNU make sees that a command to be executed contains the string
$(MAKE), it makes the jobserver that limits parallelism available to
the command.  Otherwise, any sub-make that executes sees that
parallelism is enabled but does not have access to the jobserver, so
it prints a warning and turns off parallel job execution.

This also makes the dist-docs process run the same "make" that is
executed at the top level, in case that's different from the default
"make" found in $PATH.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>

Makefile.am

@@ -408,7 +408,7 @@ if LINUX_ENABLED
 endif
 
 dist-docs:
-	VERSION=$(VERSION) $(srcdir)/build-aux/dist-docs $(srcdir) $(docs)
+	VERSION=$(VERSION) MAKE='$(MAKE)' $(srcdir)/build-aux/dist-docs $(srcdir) $(docs)
 .PHONY: dist-docs
 
 include Documentation/automake.mk

build-aux/dist-docs

@@ -44,7 +44,7 @@ rm -rf $distdir
 mkdir $distdir
 
 # Install manpages.
-make install-man mandir="$abs_distdir"/man
+${MAKE-make} install-man mandir="$abs_distdir"/man
 (cd $distdir && mv `find man -type f` . && rm -rf man)
 manpages=`cd $distdir && echo *`