[PATCH] m4/libtool.m4: fix test when multilib not used by project

test and config fails when projects do not use multilib
add usual x prefix to both sides to protect multilib test

Fixes: ab89ebbcc2ff0ecff5157982ef03627cfd615f7e

--- a/m4/libtool.m4	2024-10-03 06:50:08.000000000 -0600
+++ b/m4/libtool.m4	2024-10-19 14:09:21.384266500 -0600
@@ -2569,7 +2569,7 @@ cygwin* | mingw* | windows* | pw32* | ce
     # If user builds GCC with mulitlibs enabled,
     # it should just install on $(libdir)
     # not on $(libdir)/../bin or 32 bits dlls would override 64 bit ones.
-    if test yes = $multilib; then
+    if test xyes = x$multilib; then
     postinstall_cmds='base_file=`basename \$file`~
       dlpath=`$SHELL 2>&1 -c '\''. $dir/'\''\$base_file'\''i; echo \$dlname'\''`~
       dldir=$destdir/`dirname \$dlpath`~