README: document cmake options for system packages

Signed-off-by: Casey Bodley <cbodley@redhat.com>

diff --git a/README.md b/README.md
index c4c69185009257ca7dcb5716b36ba0ce89209b3f..96f2c6aff28d114d5cc93c9d722131be6f0cd560 100644 (file)
--- a/README.md
+++ b/README.md
@@ -98,6 +98,14 @@ for a couple of external dependencies:
        -DCMAKE_INSTALL_PREFIX=/opt/ceph -DCMAKE_C_FLAGS="-Og -g3 -gdwarf-4" \
        ..
 
+Ceph has several bundled dependencies such as Boost, RocksDB and Arrow. By
+default, cmake will build these bundled dependencies from source instead of
+using libraries that are already installed on the system. You can opt-in to
+using these system libraries, provided they meet the minimum version required
+by Ceph, with cmake options like `WITH_SYSTEM_BOOST`:
+
+       cmake -DWITH_SYSTEM_BOOST=ON [...]
+
 To view an exhaustive list of -D options, you can invoke `cmake` with:
 
        cmake -LH