Add UNITY_OUTPUT_START/COMPLETE_HEADER_DECLARATION macros (closes #799).

`UNITY_OUTPUT_CHAR` and `UNITY_OUTPUT_FLUSH` already let users provide
their own extern prototype via the matching `*_HEADER_DECLARATION` macro
(unity_internals.h:333-335 and :349-351). `UNITY_OUTPUT_START` and
`UNITY_OUTPUT_COMPLETE` did not — so users wanting to install init /
deinit hooks (e.g. serial port open/close, RTT/JTAG bring-up) could
override the macro but had no Unity-sanctioned way to declare the
function prototype.

Add the matching `_HEADER_DECLARATION` triad for both `START` and
`COMPLETE`, mirroring the FLUSH pattern verbatim. Add the corresponding
commented-out documentation lines to examples/unity_config.h so users
discover the new option alongside the existing ones.

Purely additive — the new code path only activates when the user
defines both `UNITY_OUTPUT_START` (or `_COMPLETE`) AND the matching
`*_HEADER_DECLARATION`. Default no-op behaviour is unchanged.

Author: Aditya-Chilka

examples/unity_config.h

@@ -228,8 +228,10 @@
 /* #define UNITY_OUTPUT_CHAR_HEADER_DECLARATION    RS232_putc(int) */
 /* #define UNITY_OUTPUT_FLUSH()                    RS232_flush() */
 /* #define UNITY_OUTPUT_FLUSH_HEADER_DECLARATION   RS232_flush(void) */
-/* #define UNITY_OUTPUT_START()                    RS232_config(115200,1,8,0) */
-/* #define UNITY_OUTPUT_COMPLETE()                 RS232_close() */
+/* #define UNITY_OUTPUT_START()                       RS232_config(115200,1,8,0) */
+/* #define UNITY_OUTPUT_START_HEADER_DECLARATION       RS232_config(int,int,int,int) */
+/* #define UNITY_OUTPUT_COMPLETE()                    RS232_close() */
+/* #define UNITY_OUTPUT_COMPLETE_HEADER_DECLARATION    RS232_close(void) */
 
 /* Some compilers require a custom attribute to be assigned to pointers, like
  * `near` or `far`. In these cases, you can give Unity a safe default for these

src/unity_internals.h

@@ -363,10 +363,20 @@ typedef UNITY_FLOAT_TYPE UNITY_FLOAT;
 
 #ifndef UNITY_OUTPUT_START
 #define UNITY_OUTPUT_START()
+#else
+  /* If defined as something else, make sure we declare it here so it's ready for use */
+  #ifdef UNITY_OUTPUT_START_HEADER_DECLARATION
+    extern void UNITY_OUTPUT_START_HEADER_DECLARATION;
+  #endif
 #endif
 
 #ifndef UNITY_OUTPUT_COMPLETE
 #define UNITY_OUTPUT_COMPLETE()
+#else
+  /* If defined as something else, make sure we declare it here so it's ready for use */
+  #ifdef UNITY_OUTPUT_COMPLETE_HEADER_DECLARATION
+    extern void UNITY_OUTPUT_COMPLETE_HEADER_DECLARATION;
+  #endif
 #endif
 
 #ifdef UNITY_INCLUDE_EXEC_TIME