Add a note regarding use of #if for config.h processing.

Author: mbroz

CONTRIBUTING.md

@@ -127,7 +127,8 @@ Cryptsetup uses [Linux kernel coding style](https://www.kernel.org/doc/html/late
 - Use an elaborative description in the patch header.
 - No need to use sign-off-by lines.
 - Use name prefixes (``crypt_``, ``LUKS2_`` and similar).
-- Avoid extensive preprocessor use (specifically ``#ifdef`` sections).
+- Avoid extensive preprocessor use (specifically conditional ``#if`` or ``#ifdef`` sections).
+- To check detected configuration options stored in config.h, always use ``#if SOMETHING`` (do NOT use ``#ifdef``).
 - Use output only through ``log_err, log_std, log_verbose, log_dbg`` macros.
   The ``log_dbg`` is always in English; the others should be wrapped in the ``_()`` macro for translation.
 - Use ``assert()`` but only for simple invariants and variables (avoid calling functions).