Auto merge of #28806 - wthrowe:bare-outfile-fix, r=alexcrichton

The reason this was not failing is fascinating.  The variable $(rustc)
is empty, so the make recipe was expanded as " -o foo foo.rs".  make
interpreted this as an instruction to run the command "o foo foo.rs"
and ignore any failure that occurred, because it uses a leading '-' on
a command to signal that behavior.

Author: bors

src/test/run-make/bare-outfile/Makefile

@@ -1,4 +1,6 @@
 -include ../tools.mk
 
 all:
-	$(rustc) -o foo foo.rs
+	cp foo.rs $(TMPDIR)
+	cd $(TMPDIR) && $(RUSTC) -o foo foo.rs
+	$(call RUN,foo)