make: Use GNU Make's job server when building via ninja.

`+` is necessary to mark it as recurisve[0] so that fds are passed as
expected.

Passing explicit `-j` is dropped because it takes precedence over
MAKEFLAGS and disables the job server integration.

[0]: https://www.gnu.org/software/make/manual/html_node/POSIX-Jobserver.html

make/rules-meson.mk

@@ -60,8 +60,8 @@ $$(OBJ)/.$(1)-configure$(3): $$($(2)_SRC)/meson.build
 
 $$(OBJ)/.$(1)-build$(3):
 	@echo ":: building $(3)bit $(1)..." >&2
-	env $$($(2)_ENV$(3)) \
-	ninja $$(filter -j%,$$(MAKEFLAGS)) -C "$$($(2)_OBJ$(3))" install $(-v?)
+	+env $$($(2)_ENV$(3)) \
+	ninja -C "$$($(2)_OBJ$(3))" install
 	touch $$@
 endef