Turn on strict error checks during distcheck

This would have found earlier things such as issue #6, and since it's
only done in distcheck, doesn't impact normal users (for whom
`-Werror` is a bit too strict).

Makefile

@@ -63,7 +63,7 @@ distcheck: dist
 	DST="$$BASE/dst" && mkdir "$$DST" && \
 	tar xvf $(TARFILE) -C "$$SRC" && \
 	cd "$$SRC/$(RELEASEDIR)" && \
-	make && ./mt --version && ./stinit --version && \
+	make CFLAGS="-Wall -Wextra -Werror" && ./mt --version && ./stinit --version && \
 	make dist && \
 	make install DESTDIR="$$DST" && \
 	numfiles=$$( \