sort: make the startup time optimization effective on glibc < 2.34

* configure.ac: Test where to find the dlopen function. Set LIB_DL.
Use it in the DLOPEN_LIBCRYPTO test.
* src/local.mk (src_sort_LDADD): Add $(LIB_DL).

configure.ac

@@ -351,6 +351,15 @@ if test $utils_cv_localtime_cache = yes; then
   AC_DEFINE([LOCALTIME_CACHE], [1], [FIXME])
 fi
 
+# Find the library for dynamic loading of shared libraries.
+AC_SEARCH_LIBS([dlopen], [dl])
+AS_CASE([$ac_cv_search_dlopen],
+  [no | 'none required'],
+    [LIB_DL=],
+  [*],
+    [LIB_DL="$ac_cv_search_dlopen"])
+AC_SUBST([LIB_DL])
+
 # Should 'sort' link libcrypto dynamically?
 AS_CASE([$LIB_CRYPTO],
   [-lcrypto],
@@ -360,7 +369,7 @@ AS_CASE([$LIB_CRYPTO],
        [utils_cv_dlopen_libcrypto],
        [utils_cv_dlopen_libcrypto=no
         saved_LIBS=$LIBS
-        LIBS="$LIBS $LIB_CRYPTO"
+        LIBS="$LIBS $LIB_DL $LIB_CRYPTO"
         AC_LINK_IFELSE(
           [AC_LANG_PROGRAM(
              [[#include <dlfcn.h>