fix build with download enabled

configure.ac

@@ -26,9 +26,12 @@ AC_ARG_ENABLE(pkcs11-download,
 
 if test "$download_pkcs11" != "no"; then
     AC_MSG_NOTICE([downloading pkcs11 library headers])
-    test -d usr/include || mkdir -p usr/include
-    cd usr/include
+    test -d ${prefix}/include || mkdir -p ${prefix}/include
+    cd ${prefix}/include
     for f in $(wget -qO- ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-11/v2-20/ 2>&1 | html2 | sed -n 's,.*/a/@href=\(.*\.h\)$,\1,gp'); do
+        if test -f "${f##*/}"; then
+          rm "${f##*/}"
+        fi
         wget --unlink $f
         if ! test -f "${f##*/}"; then
             AC_MSG_ERROR([download failed of pkcs11 header file: $f])