MDEV-9618 solaris sparc build fails on 10.1.

Compiler there is strict about the C/C++ call model
        mixing in function variable assumptions.
        Fixed by adding some 'extern "C"' and changing
        '?' operator with 'if'.

include/mysql/plugin_audit.h

@@ -23,6 +23,10 @@
 
 #include "plugin.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define MYSQL_AUDIT_CLASS_MASK_SIZE 1
 
 #define MYSQL_AUDIT_INTERFACE_VERSION 0x0302
@@ -175,4 +179,8 @@ struct st_mysql_audit
 };
 
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif