ExternalProject: option LOG_MERGED_STDOUTERR to combine stdout and stderr

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.