Enable certain documented defines only when generating doxygen

Avoid an "unrecognized define" error.
Signed-off-by: Andrzej Kurek <andrzej.kurek@arm.com>

doxygen/mbedtls.doxyfile

@@ -51,4 +51,5 @@ PREDEFINED             = "MBEDTLS_CHECK_RETURN_CRITICAL="   \
                          "MBEDTLS_CHECK_RETURN_TYPICAL="    \
                          "MBEDTLS_CHECK_RETURN_OPTIONAL="   \
                          "MBEDTLS_PRINTF_ATTRIBUTE(a,b)="   \
+                         "__DOXYGEN__"                      \
 

include/mbedtls/mbedtls_config.h

@@ -3711,6 +3711,7 @@
  * MBEDTLS_PLATFORM_STD_CALLOC and MBEDTLS_PLATFORM_STD_FREE do not have to be defined at the same time, as, if they are used,
  * dynamic setup of these functions is possible. See the tree above to see how are they handled in all cases.
  */
+
 /** \def MBEDTLS_PLATFORM_STD_CALLOC
  *
  * Default allocator to use, can be undefined. See the description above for details.
@@ -3721,6 +3722,7 @@
  * The corresponding deallocation function is #MBEDTLS_PLATFORM_STD_FREE.
  */
 //#define MBEDTLS_PLATFORM_STD_CALLOC        calloc
+
 /** \def MBEDTLS_PLATFORM_STD_FREE
  *
  * Default free to use, can be undefined. See the description above for more details (same principles as for MBEDTLS_PLATFORM_STD_CALLOC apply).

include/mbedtls/platform.h

@@ -130,6 +130,15 @@ extern "C" {
 #endif
 #endif /* MBEDTLS_PLATFORM_NO_STD_FUNCTIONS */
 
+/* Enable certain documented defines only when generating doxygen to avoid
+ * an "unrecognized define" error. */
+#if defined(__DOXYGEN__) && !defined(MBEDTLS_PLATFORM_STD_CALLOC)
+#define MBEDTLS_PLATFORM_STD_CALLOC
+#endif
+
+#if defined(__DOXYGEN__) && !defined(MBEDTLS_PLATFORM_STD_FREE)
+#define MBEDTLS_PLATFORM_STD_FREE
+#endif
 
 /** \} name SECTION: Module settings */