Most steps support the logging into a file but the patch command is a
notable exception. This commit adds the LOG_PATCH options that acts as
the other LOG_* options.
In some circumstances the user of ExternalProject may not desire the
split log files for stdout and stderr. In particular with a project has
many errors it can be difficult to correlate the output error with the
command that it resulted from.
This commit adds the LOG_MERGED_STDOUTERR option that when enabled
outputs into a unified <name>-<step>.log for each step. If disabled it
will default to the previous behaviour of <name>-<step>-out.log and
<name>-<step>-err.log.
