configure: sort of fix speexdsp

$enable_speex can't be empty at this point so this check makes no sense
(cherry picked from commit c6184580)
Signed-off-by: Pierre Ynard <linkfanel@yahoo.fr>

configure.ac

@@ -2814,11 +2814,7 @@ AS_IF([test "${enable_speex}" != "no"], [
   PKG_CHECK_MODULES([SPEEXDSP], [speexdsp], [
     have_speexdsp="yes"
   ], [
-    AS_IF([test -n "${enable_speex}"], [
-      AC_MSG_ERROR([${SPEEXDSP_PKG_ERRORS}.])
-    ], [
-      AC_MSG_WARN([${SPEEXDSP_PKG_ERRORS}.])
-    ])
+    AC_MSG_WARN([${SPEEXDSP_PKG_ERRORS}.])
   ])
 ])
 AM_CONDITIONAL([HAVE_SPEEXDSP], [test "$have_speexdsp" = "yes"])