Fix source dir detection in bootstrap.sh (boostorg#162)

Author: dreifachstein

bootstrap.sh

@@ -188,8 +188,7 @@ EOF
 fi
 test -n "$want_help" && exit 0
 
-# TBD: Determine where the script is located
-my_dir="."
+my_dir=$(dirname "$0")
 
 # Determine the toolset, if not already decided
 if test "x$TOOLSET" = x; then