gperf: fix out-of-tree builds

(From OE-Core rev: 74d853525fc4252f717f0baaa322e6aa5c4eb88f)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-extended/gperf/gperf.inc

@@ -14,8 +14,8 @@ inherit autotools
 EXTRA_AUTORECONF += " -I ${S}"
 
 do_configure_prepend() {
-        if [ ! -e acinclude.m4 ]; then
-                cat aclocal.m4 > acinclude.m4
+        if [ ! -e ${S}/acinclude.m4 ]; then
+                cat ${S}/aclocal.m4 > ${S}/acinclude.m4
         fi
 }