doc: add with_bdb to unix build docs

Author: tdb3

doc/build-freebsd.md

@@ -118,7 +118,7 @@ Run `cmake -B build -LH` to see the full list of available options.
 This enables support for both wallet types, assuming
 `sqlite3` and `db4` are both installed.
 ```bash
-cmake -B build -DBerkeleyDB_INCLUDE_DIR:PATH="${BDB_PREFIX}/include"
+cmake -B build -DBerkeleyDB_INCLUDE_DIR:PATH="${BDB_PREFIX}/include" -DWITH_BDB=ON
 ```
 
 ##### No Wallet or GUI

doc/build-openbsd.md

@@ -112,7 +112,7 @@ Run `cmake -B build -LH` to see the full list of available options.
 This enables support for both wallet types:
 
 ```bash
-cmake -B build -DBerkeleyDB_INCLUDE_DIR:PATH="${BDB_PREFIX}/include"
+cmake -B build -DBerkeleyDB_INCLUDE_DIR:PATH="${BDB_PREFIX}/include" -DWITH_BDB=ON
 ```
 
 ### 2. Compile

doc/build-unix.md

@@ -162,7 +162,7 @@ and configure using the following:
 ```bash
 export BDB_PREFIX="/path/to/bitcoin/depends/x86_64-pc-linux-gnu"
 
-cmake -B build -DBerkeleyDB_INCLUDE_DIR:PATH="${BDB_PREFIX}/include"
+cmake -B build -DBerkeleyDB_INCLUDE_DIR:PATH="${BDB_PREFIX}/include" -DWITH_BDB=ON
 ```
 
 **Note**: Make sure that `BDB_PREFIX` is an absolute path.