[build] Add db4_cxx to bitcoin_find_bdb48.m4

Added "db4_cxx" as this is the name used in NetBSD for the Berkeley DB library.  Without this additional base filename alternative, the "configure" script will fail to find this library and compilation will not succeed.

build-aux/m4/bitcoin_find_bdb48.m4

@@ -64,7 +64,7 @@ AC_DEFUN([BITCOIN_FIND_BDB48],[
   
   if test "x$BDB_LIBS" = "x"; then
     # TODO: Ideally this could find the library version and make sure it matches the headers being used
-    for searchlib in db_cxx-4.8 db_cxx; do
+    for searchlib in db_cxx-4.8 db_cxx db4_cxx; do
       AC_CHECK_LIB([$searchlib],[main],[
         BDB_LIBS="-l${searchlib}"
         break