Provide network download fallback

in case the base infrastructure for providing the networks

https://tests.stockfishchess.org/nns

is down, use an alternate github repo for downloading networks during the build.

fixes #4149
fixes #4140

closes #4151

No functional change

src/Makefile

@@ -827,24 +827,34 @@ clean: objclean profileclean
 net:
 	$(eval nnuenet := $(shell grep EvalFileDefaultName evaluate.h | grep define | sed 's/.*\(nn-[a-z0-9]\{12\}.nnue\).*/\1/'))
 	@echo "Default net: $(nnuenet)"
-	$(eval nnuedownloadurl := https://tests.stockfishchess.org/api/nn/$(nnuenet))
+	$(eval nnuedownloadurl1 := https://tests.stockfishchess.org/api/nn/$(nnuenet))
+	$(eval nnuedownloadurl2 := https://github.com/official-stockfish/networks/raw/master/$(nnuenet))
 	$(eval curl_or_wget := $(shell if hash curl 2>/dev/null; then echo "curl -skL"; elif hash wget 2>/dev/null; then echo "wget -qO-"; fi))
-	@if test -f "$(nnuenet)"; then \
-		echo "Already available."; \
-	else \
-		if [ "x$(curl_or_wget)" = "x" ]; then \
-			echo "Automatic download failed: neither curl nor wget is installed. Install one of these tools or download the net manually"; exit 1; \
-		else \
-			echo "Downloading $(nnuedownloadurl)"; $(curl_or_wget) $(nnuedownloadurl) > $(nnuenet);\
-		fi; \
-	fi;
+	@if [ "x$(curl_or_wget)" = "x" ]; then \
+	    echo "Automatic download failed: neither curl nor wget is installed. Install one of these tools or download the net manually"; exit 1; \
+        fi
 	$(eval shasum_command := $(shell if hash shasum 2>/dev/null; then echo "shasum -a 256 "; elif hash sha256sum 2>/dev/null; then echo "sha256sum "; fi))
-	@if [ "x$(shasum_command)" != "x" ]; then \
-		if [ "$(nnuenet)" != "nn-"`$(shasum_command) $(nnuenet) | cut -c1-12`".nnue" ]; then \
-			echo "Failed download or $(nnuenet) corrupted, please delete!"; exit 1; \
-		fi \
-	else \
-		echo "shasum / sha256sum not found, skipping net validation"; \
+	@if [ "x$(shasum_command)" = "x" ]; then \
+            echo "shasum / sha256sum not found, skipping net validation"; \
+        fi
+	@for nnuedownloadurl in "$(nnuedownloadurl1)" "$(nnuedownloadurl2)"; do \
+	   if test -f "$(nnuenet)"; then \
+	      echo "$(nnuenet) available."; \
+	   else \
+	      if [ "x$(curl_or_wget)" != "x" ]; then \
+	         echo "Downloading $${nnuedownloadurl}"; $(curl_or_wget) $${nnuedownloadurl} > $(nnuenet);\
+	      fi; \
+	   fi; \
+	   if [ "x$(shasum_command)" != "x" ]; then \
+	      if [ "$(nnuenet)" != "nn-"`$(shasum_command) $(nnuenet) | cut -c1-12`".nnue" ]; then \
+	         echo "Removing failed download"; rm -f $(nnuenet); \
+	      else \
+	         echo "Network validated"; break; \
+	      fi; \
+	   fi; \
+	done
+	@if ! test -f "$(nnuenet)"; then \
+	    echo "Failed to download $(nnuenet)."; \
 	fi
 
 # clean binaries and objects