fail when the C compiler fails, and clean up when we do

compile-to-file returned process-wait's values and main dropped them,
so cc errors were printed, but then main compiler exited 0.

Also cleanup tmp C files when compilation failed.

tests/exit-code/Makefile

@@ -0,0 +1,26 @@
+# The failure paths.
+#
+# Both need a process to show themselves, so neither fits the unit
+# suite: sexc has to fail when cc fails -- exiting 0 after a failed
+# compile makes every driver, sextest included, read it as success --
+# and it has to clean up its temporary .c when its own emission throws.
+#
+# Everything is built inside a scratch TMPDIR, so nothing is left here
+# to clean up.
+
+SEXC ?= ../../sexc
+
+check:
+	@d=`mktemp -d`; \
+	TMPDIR=$$d $(SEXC) nested-pointer.sex -o $$d/out >/dev/null 2>&1; \
+	if [ -n "`find $$d -name '*.c'`" ]; then \
+		echo "exit code FAILED: the temporary .c survived a failed emission"; \
+		rm -rf $$d; exit 1; \
+	fi; \
+	if $(SEXC) hello.sex -o $$d/out -- -no-such-cc-flag >/dev/null 2>&1; then \
+		echo "exit code FAILED: sexc reported success after cc failed"; \
+		rm -rf $$d; exit 1; \
+	fi; \
+	rm -rf $$d; echo "exit code ok"
+
+.PHONY: check

tests/exit-code/hello.sex

@@ -0,0 +1,4 @@
+;;; Compiles cleanly, so the only way the build can fail is the bogus
+;;; flag handed to cc -- which is the point.
+(pub fn main () int
+  (return 0))

tests/exit-code/nested-pointer.sex

@@ -0,0 +1,4 @@
+;;; Rejected by the writer, so emit-c throws: there is a temporary .c
+;;; by then, and it must not survive.
+(fn f () void
+  (var p (* (* char))))