libm4: spell TRUNCATE_FILENAME consistently.

* m4/path.c (m4_path_search): s/FILE_TRUNCATE/TRUNCATE_FILENAME/
to match definition.
Reported by KO Myung-Hun

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
diff --git a/m4/path.c b/m4/path.c
index e2816cf..ef743a3 100644
--- a/m4/path.c
+++ b/m4/path.c
@@ -225,7 +225,7 @@
 
       /* Try appending each of the suffixes we were given.  */
       filepath = strncpy (xmalloc (mem + max_suffix_len +1), filename, mem +1);
-#if FILE_TRUNCATE
+#if TRUNCATE_FILENAME
       filepath = path_truncate (filepath);
       mem = strlen (filepath); /* recalculate length after truncation */
 #endif
@@ -271,7 +271,7 @@
 
       filepath = strncpy (xmalloc (mem + max_suffix_len +1), pathname, mem +1);
       free (pathname);
-#if FILE_TRUNCATE
+#if TRUNCATE_FILENAME
       filepath = path_truncate (filepath);
       mem = strlen (filepath); /* recalculate length after truncation */
 #endif