Fix CURL_CHECK_LIBS_LDAP failure when no parameter is given

acinclude.m4

@@ -492,6 +492,8 @@ AC_DEFUN([CURL_CHECK_LIBS_LDAP], [
   AC_REQUIRE([CURL_CHECK_HEADER_LDAP])dnl
   #
   u_libs=""
+  #
+  ifelse($1,,,[
     for x_lib in $1; do
       case "$x_lib" in
         -l*)
@@ -507,6 +509,7 @@ AC_DEFUN([CURL_CHECK_LIBS_LDAP], [
         u_libs="$u_libs $l_lib"
       fi
     done
+  ])
   #
   AC_MSG_CHECKING([for LDAP libraries])
   #