libxml.h: Add comments and indentation

libxml.h

@@ -23,13 +23,17 @@
 #endif
 
 /*
- * Currently supported platforms use either autoconf or
- * copy to config.h own "preset" configuration file.
- * As result ifdef HAVE_CONFIG_H is omitted here.
+ * These files are generated by the build system and contain private
+ * and public build configuration.
  */
 #include "config.h"
 #include <libxml/xmlversion.h>
 
+/*
+ * Due to some Autotools limitations, this variable must be passed as
+ * compiler flag. Define a default value if the macro wasn't set by the
+ * build system.
+ */
 #ifndef SYSCONFDIR
   #define SYSCONFDIR "/etc"
 #endif