ExternalProject: add LOG_DIR option that allows overriding of log location

In some situations it can be helpful to separate out the location of the
log files from the location of the stamp files. For instance if you have
a continuous integration that exposes the location where log files are
stored.

This commit adds an option that allows a user to override the default
behaviour of putting the log files in STAMP_DIR called LOG_DIR. The
previous behaviour of putting the log files into the STAMP_DIR applies
if LOG_DIR is not specified.