Install script: fall back to su without sudo, add --build-only

Change-Id: Ibfbaef4ee01a35a0bd8f430d423746f25939b10c
diff --git a/scripts/build-rocksdb.sh b/scripts/build-rocksdb.sh
index cc2cd1f..fca1e44 100755
--- a/scripts/build-rocksdb.sh
+++ b/scripts/build-rocksdb.sh
@@ -78,7 +78,7 @@
 if [ -n "$missing" ]; then
     echo "Missing build tools:$missing" >&2
     echo >&2
-    echo "Install them and the compression libraries:" >&2
+    echo "Install them and the compression libraries, with sudo or su -c:" >&2
     echo "  Fedora, Rocky, RHEL: sudo dnf install cmake gcc-c++ git snappy-devel zlib-devel bzip2-devel lz4-devel libzstd-devel" >&2
     echo "  Debian, Ubuntu:      sudo apt-get install cmake g++ git libsnappy-dev zlib1g-dev libbz2-dev liblz4-dev libzstd-dev" >&2
     echo "  MacOS:               brew install cmake snappy zlib bzip2 lz4 zstd" >&2
@@ -151,4 +151,5 @@
     echo
     echo "Programs that use the shared library find it after:"
     echo "  echo $PREFIX/$LIBDIR | sudo tee /etc/ld.so.conf.d/rocksdb.conf && sudo ldconfig"
+    echo "  (or 'su -c \"...\"' where there is no sudo)"
 fi