Remove config.h from public headers and include it in non-header files instead

To avoid symbol collision such as PACKAGE or VERSION with
autotools-based project.
diff --git a/crypto/math/stat.c b/crypto/math/stat.c
index 5e46c20..84cdf2f 100644
--- a/crypto/math/stat.c
+++ b/crypto/math/stat.c
@@ -7,6 +7,10 @@
  * Cisco Systems, Inc.
  */
 
+#ifdef HAVE_CONFIG_H
+    #include <config.h>
+#endif
+
 #include "stat.h"
 
 debug_module_t mod_stat = {