Add configured includedir as a default import path

Protected by an ifdef to avoid tripping up non-autotools builds
Support for exception backtraces on OS X.
Merge pull request #109 from greedy/add-includedir-to-imports

Add configured includedir as a default import path
Consolidate Linux and OS X versions of getStackSymbols().
Replace 'goto' error handling with KJ_DEFER.
Merge pull request #111 from partylemon/ExceptionBacktraces

Support for printing exception backtrace symbols on OS X.