Litecoin: Make it easier to specify gbuild resource limits

Add variables for gbuild thread and memory limits.  Simplifies using custom values.

doc/release-process.md

@@ -140,17 +140,20 @@ The gbuild invocations below <b>DO NOT DO THIS</b> by default.
 
 ### Build and sign Litecoin Core for Linux, Windows, and macOS:
 
+    export GITIAN_THREADS=2
+    export GITIAN_MEMORY=3000
+    
     pushd ./gitian-builder
-    ./bin/gbuild --num-make 2 --memory 3000 --commit litecoin=v${VERSION} ../litecoin/contrib/gitian-descriptors/gitian-linux.yml
+    ./bin/gbuild --num-make $GITIAN_THREADS --memory $GITIAN_MEMORY --commit litecoin=v${VERSION} ../litecoin/contrib/gitian-descriptors/gitian-linux.yml
     ./bin/gsign --signer "$SIGNER" --release ${VERSION}-linux --destination ../gitian.sigs.ltc/ ../litecoin/contrib/gitian-descriptors/gitian-linux.yml
     mv build/out/litecoin-*.tar.gz build/out/src/litecoin-*.tar.gz ../
 
-    ./bin/gbuild --num-make 2 --memory 3000 --commit litecoin=v${VERSION} ../litecoin/contrib/gitian-descriptors/gitian-win.yml
+    ./bin/gbuild --num-make $GITIAN_THREADS --memory $GITIAN_MEMORY --commit litecoin=v${VERSION} ../litecoin/contrib/gitian-descriptors/gitian-win.yml
     ./bin/gsign --signer "$SIGNER" --release ${VERSION}-win-unsigned --destination ../gitian.sigs.ltc/ ../litecoin/contrib/gitian-descriptors/gitian-win.yml
     mv build/out/litecoin-*-win-unsigned.tar.gz inputs/litecoin-win-unsigned.tar.gz
     mv build/out/litecoin-*.zip build/out/litecoin-*.exe ../
 
-    ./bin/gbuild --num-make 2 --memory 3000 --commit litecoin=v${VERSION} ../litecoin/contrib/gitian-descriptors/gitian-osx.yml
+    ./bin/gbuild --num-make $GITIAN_THREADS --memory $GITIAN_MEMORY --commit litecoin=v${VERSION} ../litecoin/contrib/gitian-descriptors/gitian-osx.yml
     ./bin/gsign --signer "$SIGNER" --release ${VERSION}-osx-unsigned --destination ../gitian.sigs.ltc/ ../litecoin/contrib/gitian-descriptors/gitian-osx.yml
     mv build/out/litecoin-*-osx-unsigned.tar.gz inputs/litecoin-osx-unsigned.tar.gz
     mv build/out/litecoin-*.tar.gz build/out/litecoin-*.dmg ../