build_rocksdb.sh: use the NPROC env var for number of make jobs

scripts/build_rocksdb.sh

@@ -16,6 +16,10 @@ ROCKSDBVER="5.17.2"
 [[ $# -ne 1 ]] && { echo "usage: $0 ci_cache_dir"; exit 1; }
 CI_CACHE="$1" # here we cache the installed files
 
+# env vars
+
+[[ -z "$NPROC" ]] && NPROC=2 # number of CPU cores available
+
 # install from cache and exit, if the version we want is already there
 if [[ -n "$CI_CACHE" ]] && ls "$CI_CACHE"/lib/librocksdb* 2>/dev/null | grep -q "$ROCKSDBVER"; then
 	sudo cp -a "$CI_CACHE"/* /usr/local/
@@ -24,10 +28,10 @@ fi
 
 # build it
 echo "Building RocksDB"
-curl -O -L -s -S https://github.com/facebook/rocksdb/archive/v$ROCKSDBVER.tar.gz
-tar xzf v$ROCKSDBVER.tar.gz
-cd rocksdb-$ROCKSDBVER
-make DISABLE_WARNING_AS_ERROR=1 -j2 shared_lib
+curl -O -L -s -S https://github.com/facebook/rocksdb/archive/v${ROCKSDBVER}.tar.gz
+tar xzf v${ROCKSDBVER}.tar.gz
+cd rocksdb-${ROCKSDBVER}
+make DISABLE_WARNING_AS_ERROR=1 -j${NPROC} shared_lib
 
 # install it
 if [[ -n "../$CI_CACHE" ]]; then