Fix $ECHO abuse exposed by recent patch.

[Ralf Wildenhues]

The $ECHO changes (now using printf on GNU/Linux) exposed a buglet,
fixed as below.

Cheers,
Ralf

    Fix $ECHO abuse exposed by recent patch.
    
    * libltdl/m4/libtool.m4 (_LT_SYS_DYNAMIC_LINKER): Require
    _LT_CHECK_SHELL_FEATURES, for $lt_NL2SP.  Always quote argument
    to $ECHO.

diff --git a/libltdl/m4/libtool.m4 b/libltdl/m4/libtool.m4
index a51c47d..763a410 100644
--- a/libltdl/m4/libtool.m4
+++ b/libltdl/m4/libtool.m4
@@ -1954,6 +1954,7 @@ m4_require([_LT_DECL_EGREP])dnl
 m4_require([_LT_FILEUTILS_DEFAULTS])dnl
 m4_require([_LT_DECL_OBJDUMP])dnl
 m4_require([_LT_DECL_SED])dnl
+m4_require([_LT_CHECK_SHELL_FEATURES])dnl
 AC_MSG_CHECKING([dynamic linker characteristics])
 m4_if([$1],
        [], [
@@ -1984,7 +1985,7 @@ if test "$GCC" = yes; then
        lt_tmp_lt_search_path_spec="$lt_tmp_lt_search_path_spec $lt_sys_path"
     fi
   done
-  lt_search_path_spec=`$ECHO $lt_tmp_lt_search_path_spec | awk '
+  lt_search_path_spec=`$ECHO "$lt_tmp_lt_search_path_spec" | awk '
 BEGIN {RS=" "; FS="/|\n";} {
   lt_foo="";
   lt_count=0;
@@ -2004,7 +2005,7 @@ BEGIN {RS=" "; FS="/|\n";} {
   if (lt_foo != "") { lt_freq[[lt_foo]]++; }
   if (lt_freq[[lt_foo]] == 1) { print lt_foo; }
 }'`
-  sys_lib_search_path_spec=`$ECHO $lt_search_path_spec`
+  sys_lib_search_path_spec=`$ECHO "$lt_search_path_spec" | $lt_NL2SP`
 else
   sys_lib_search_path_spec="/lib /usr/lib /usr/local/lib"
 fi])