doc: swap CPPFLAGS for APPEND_CPPFLAGS

APPEND_CPPFLAGS will be understood by our CMake, whereas CPPFLAGS will
not.

Author: fanquake

doc/developer-notes.md

@@ -432,8 +432,8 @@ RPC that, when enabled, logs the location and duration of each lock contention
 to the `debug.log` file.
 
 The `-DCMAKE_BUILD_TYPE=Debug` build option adds `-DDEBUG_LOCKCONTENTION` to the
-compiler flags. You may also enable it manually by building with `-DDEBUG_LOCKCONTENTION` added to your CPPFLAGS,
-i.e. `CPPFLAGS="-DDEBUG_LOCKCONTENTION"`, then build and run bitcoind.
+compiler flags. You may also enable it manually by building with `-DDEBUG_LOCKCONTENTION`
+added to your CPPFLAGS, i.e. `-DAPPEND_CPPFLAGS="-DDEBUG_LOCKCONTENTION"`.
 
 You can then use the `-debug=lock` configuration option at bitcoind startup or
 `bitcoin-cli logging '["lock"]'` at runtime to turn on lock contention logging.