Makefile: Switch from md5sums to sha256sums

We switched to these several stable releases ago, (since the MD5 algorithm has
been broken for some time), but only now did I get around to fixing this in
the Makefile rather than just performing this step manually.

CC: "10.2 10.3" <mesa-stable@lists.freedesktop.org>
diff --git a/Makefile.am b/Makefile.am
index 4780510..d2916da 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -71,7 +71,7 @@
 	$(PACKAGE_NAME).tar.bz2 \
 	$(PACKAGE_NAME).zip
 
-tarballs: md5
+tarballs: checksums
 	rm -f ../$(PACKAGE_DIR) $(PACKAGE_NAME).tar
 
 manifest.txt: .git
@@ -98,9 +98,9 @@
 	zip -q -@ $(PACKAGE_NAME).zip < $(PACKAGE_DIR)/manifest.txt ; \
 	mv $(PACKAGE_NAME).zip $(PACKAGE_DIR)
 
-md5: $(ARCHIVES)
-	@-md5sum $(PACKAGE_NAME).tar.gz
-	@-md5sum $(PACKAGE_NAME).tar.bz2
-	@-md5sum $(PACKAGE_NAME).zip
+checksums: $(ARCHIVES)
+	@-sha256sum $(PACKAGE_NAME).tar.gz
+	@-sha256sum $(PACKAGE_NAME).tar.bz2
+	@-sha256sum $(PACKAGE_NAME).zip
 
 .PHONY: tarballs md5