Bug 1367940 - Pass the make jobserver file descriptors down to cargo. r=gps

cargo 1.19, and later cargo 1.20/rustc 1.19 gained support for the make
jobserver. But that doesn't work unless make passes the jobserver file
descriptors down to subprocesses, which it only does when there is a +
prefixing commands.

--HG--
extra : rebase_source : a00ffb7edb358243ac7b9469941ec6fb5ad84885

config/rules.mk

@@ -913,8 +913,11 @@ else
 rust_debug_info=2
 endif
 
+# We use the + prefix to pass down the jobserver fds to cargo, but we
+# don't use the prefix when make -n is used, so that cargo doesn't run
+# in that case)
 define RUN_CARGO
-env $(environment_cleaner) $(rust_unlock_unstable) $(rustflags_override) $(sccache_wrap) \
+$(if $(findstring n,$(filter-out --%, $(MAKEFLAGS))),,+)env $(environment_cleaner) $(rust_unlock_unstable) $(rustflags_override) $(sccache_wrap) \
 	CARGO_TARGET_DIR=$(CARGO_TARGET_DIR) \
 	RUSTC=$(RUSTC) \
 	RUSTFLAGS='-C debuginfo=$(rust_debug_info)' \