travis: cache leveldb artifacts to reduce build time

tests/build-leveldb.sh

@@ -0,0 +1,24 @@
+#!/bin/bash
+
+set -e
+
+LEVELDB_VERSION="$1"
+if [ ! -f "./leveldb-mcpe/built_version" ] || [ $(cat "./leveldb-mcpe/built_version") != "$LEVELDB_VERSION" ]; then
+    echo "Building new LevelDB"
+    rm -rf "./leveldb-mcpe" || true
+    mkdir "./leveldb-mcpe"
+
+    curl -fsSL "https://github.com/pmmp/leveldb-mcpe/archive/$LEVELDB_VERSION.tar.gz" | tar -zx
+    mv "./leveldb-mcpe-$LEVELDB_VERSION" leveldb-mcpe-build
+    cd leveldb-mcpe-build
+    make -j4 sharedlibs
+
+    #put the artifacts in a separate dir, to avoid bloating travis cache"
+    mv out-shared/libleveldb.* ../leveldb-mcpe
+    mv include ../leveldb-mcpe
+    cd ../leveldb-mcpe
+    echo "$LEVELDB_VERSION" > "./built_version"
+    cd ..
+else
+    echo "Using cached build for LevelDB version $LEVELDB_VERSION"
+fi