Add API to get callbacks for log messages

Useful when wrapping KSCrash in a framework that has other ways to
log than stdout/files.

Will not affect file/stdout logging, so that those can be toggled
independently.

Source/KSCrash/Recording/Tools/KSLogger.c

@@ -272,12 +272,23 @@ void kslog_setLogToStdout(bool enabled)
 #pragma mark - C -
 // ===========================================================================
 
+static KSLogFunction g_logCallback = NULL;
+
+void kslog_setLogCallback(KSLogFunction function)
+{
+    g_logCallback = function;
+}
+
 static void i_kslog_log(const char* const level,
                         const char* const file,
                         const int line,
                         const char* const function,
                         const char* const fmt, va_list args)
 {
+    if (g_logCallback)
+    {
+        g_logCallback(level, file, line, function, fmt, args);
+    }
     if(level && file && line >= 0 && function)
     {
         writeFmtToLog("%s: %s (%u): %s: ", level, lastPathEntry(file), line, function);

Source/KSCrash/Recording/Tools/KSLogger.h

@@ -153,6 +153,7 @@ extern "C" {
 
 
 #include <stdbool.h>
+#include <stdarg.h>
 
 
 #ifdef __OBJC__
@@ -267,6 +268,22 @@ bool kslog_clearLogFile();
  */
 void kslog_setLogToStdout(bool enabled);
 
+typedef void (*KSLogFunction)(const char* level, const char* file,
+                     int line, const char* function,
+                     const char* fmt, va_list args);
+
+/** Set a function responsible for logging.
+ *
+ * When set, will deliver logging to this function
+ * in addition to file and stdout output (if enabled).
+ *
+ * WARNING: Only call async-safe functions from this function!
+ * DO NOT call Objective-C methods!!!
+ *
+ * @param function The function that should handle logging.
+ */
+void kslog_setLogCallback(KSLogFunction function);
+
 /** Tests if the logger would print at the specified level.
  *
  * @param LEVEL The level to test for. One of: