Update build-unix.md

If a user hasn't ready run autogen.sh, configure doesn't yet exist.  Do the instructions seem to presume you have already built bitcoin?  I don't think they do.

Would stifle beginners.

Update build-unix.md

doc/build-unix.md

@@ -154,7 +154,8 @@ make install
 
 # Configure Bitcoin Core to use our own-built instance of BDB
 cd $BITCOIN_ROOT
-./configure (other args...) LDFLAGS="-L${BDB_PREFIX}/lib/" CPPFLAGS="-I${BDB_PREFIX}/include/"
+./autogen.sh
+./configure LDFLAGS="-L${BDB_PREFIX}/lib/" CPPFLAGS="-I${BDB_PREFIX}/include/" # (other args...)
 ```
 
 **Note**: You only need Berkeley DB if the wallet is enabled (see the section *Disable-Wallet mode* below).