Introduce internal includes section to avoid potential implicit declaration warnings

[ycherniavskyi]

During the integration of the libcanard into a project based on ChibiOS I want to change CANARD_ASSERT from the default assert to ChibiOS osalDbgCheck. It is done with the next C defines -DCANARD_ASSERT=osalDbgCheck -DCANARD_CONFIG_HEADER=\"${BINDINGS_DIR}/canard/canard_config.h\", there to prevent implicit declaration warnings I unutilized CANARD_CONFIG_HEADER by providing a canard_config.h file with the following content:

#include "osal.h"

But because of #include "_canard_cavl.h" and # include CANARD_CONFIG_HEADER order in canard.c I still get implicit declaration warnings:

Compiling canard.c
../../ext/OpenCyphal/libcanard/libcanard/_canard_cavl.h: In function 'cavlPrivateRotate':
<command-line>: warning: implicit declaration of function 'osalDbgCheck' [-Wimplicit-function-declaration]
../../ext/OpenCyphal/libcanard/libcanard/_canard_cavl.h:30:25: note: in expansion of macro 'CANARD_ASSERT'
   30 | #    define CAVL_ASSERT CANARD_ASSERT
      |                         ^~~~~~~~~~~~~
../../ext/OpenCyphal/libcanard/libcanard/_canard_cavl.h:98:5: note: in expansion of macro 'CAVL_ASSERT'
   98 |     CAVL_ASSERT((x != NULL) && (x->lr[!r] != NULL) && ((x->bf >= -1) && (x->bf <= +1)));
      |     ^~~~~~~~~~~

To resolve this issue # include CANARD_CONFIG_HEADER must be placed before #include "_canard_cavl.h".

[pavel-kirienko]

Thank you. At first, I thought it violates AUTOSAR C++ M16-0-1, but upon closer inspection, I see that this solution is compliant.

I tried to push the following corrections before merging but your fork is not writable for me; please apply them yourself and request another review:

diff --git a/README.md b/README.md
index 4ebc819..f651750 100644
--- a/README.md
+++ b/README.md
@@ -241,6 +241,11 @@ If you find the examples to be unclear or incorrect, please, open a ticket.
 
 - Refactor the transfer reassembly state machine to enhance its maintainability and robustness.
 
+#### v3.1.2
+
+- Allow redefinition of CANARD_ASSERT via the config header;
+  see [#219](https://github.com/OpenCyphal/libcanard/pull/219).
+
 ### v3.0
 
 - Update branding as [UAVCAN v1 is renamed to Cyphal](https://forum.opencyphal.org/t/uavcan-v1-is-now-cyphal/1622).
diff --git a/libcanard/canard.c b/libcanard/canard.c
index 5cd28aa..863de50 100644
--- a/libcanard/canard.c
+++ b/libcanard/canard.c
@@ -38,6 +38,7 @@
 #endif
 
 // --------------------------------------------- INTERNAL INCLUDES ----------------------------------------------
+// The internal includes are placed here after the config header is included and CANARD_ASSERT is defined.
 
 #include "_canard_cavl.h"