Fix cd error when running bjam from non-top

bjam

@@ -1,17 +1,17 @@
 #!/bin/bash
 set -e
+top="$(dirname "$0")"
 if
   bjam="$(which bjam 2>/dev/null)" && #exists
   [ ${#bjam} != 0 ] && #paranoia about which printing nothing then returning true
   ! grep UFIHGUFIHBDJKNCFZXAEVA "${bjam}" </dev/null >/dev/null && #bjam in path isn't this script
   "${bjam}" --sanity-test 2>/dev/null |grep Sane >/dev/null && #The test in jam-files/sanity.jam passes
-  (cd jam-files/fail && ! "${bjam}") >/dev/null #Returns non-zero on failure
+  (cd "${top}/jam-files/fail" && ! "${bjam}") >/dev/null #Returns non-zero on failure
 then
   #Delegate to system bjam
   exec "${bjam}" "$@"
 fi
 
-top="$(dirname "$0")"
 if [ ! -x "$top"/jam-files/bjam ] || "$top"/jam-files/bjam -v |grep 2011.4 >/dev/null; then
   pushd "$top/jam-files/engine"
   ./build.sh