bug#21076: dynamic-link

I have encountered this issue with dynamic-link.
I propose a small patch to Guile (works on v3.0.1) to address it.
At least this allows me to load the file if I know the exact name.

This patch fixes issue with libtool and OSes which use
non-standard extensions for shared libraries.  The libltdl
function lt_dlopenext expects the argument to have a standard
extension or no extention.  Arguments reflecting shared object
files with non-standard extensions will fail to load.  For
example, my system has /usr/lib64/libyaml-0.so.2.  w/o this
patch (dynamic-link "libyaml-0.so.2") does not work; with it,
it does.  Go figure.

--- libguile/dynl.c-orig    2020-03-20 05:56:42.101214929 -0700
+++ libguile/dynl.c    2020-03-20 05:57:40.432893330 -0700
@@ -69,7 +69,10 @@
      handle = lt_dlopen (NULL);
    else
      {
-      handle = lt_dlopenext (fname);
+      handle = lt_dlopen (fname);
+
+      if (handle == NULL)
+         handle = lt_dlopenext (fname);

        if (handle == NULL
  #ifdef LT_DIRSEP_CHAR