Added support for cmake variable HT_DEPENDENCY_DIR

[Doug Judd]

The HT_DEPENDENCY_DIR specifies a directory where cmake can find all of the dependencies.  Adding this variable was necessary to build a recent version of gcc to compile Hypertble with, without wreaking havoc on the existing system.

http://github.com/nuggetwheat/hypertable/commit/43400ee157ae4fdc9b2a940810aa9b815e9edb3d

- Doug