Re-try Minisat download (up to two times) in case it fails

Running in CI we do see transient network access errors from time to time, which
should not result in task failures (as long as they are genuinely transient).

Author: tautschnig

src/Makefile

@@ -89,7 +89,12 @@ TAR = tar
 
 minisat2-download:
 	@echo "Downloading Minisat 2.2.1"
-	@$(DOWNLOADER) http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz
+	@for i in `seq 1 3` ; do \
+	  $(DOWNLOADER) \
+	    http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz && \
+	    exit 0 ; \
+	  sleep 10 ; \
+	done ; exit 1
 	@$(TAR) xfz minisat2_2.2.1.orig.tar.gz
 	@rm -Rf ../minisat-2.2.1
 	@mv minisat2-2.2.1 ../minisat-2.2.1