Allow the mppdepth setting to be omitted in launchcmd script.

When CHPL_PBSPRO_NO_MPPDEPTH is set in the environment, the mppdepth argument
will be omitted when calling qsub.
Merge branch 'master' of github.com:chapel-lang/chapel into pbs-launchcmd-fix
Add comment explaining special case in launchcmd script.

per review by @ronawho
Merge pull request #297 from thomasvandoren/pbs-launchcmd-fix

Allow the mppdepth setting to be omitted in launchcmd script.

When CHPL_PBSPRO_NO_MPPDEPTH is set in the environment, the mppdepth argument
will be omitted when calling qsub.

reviewed by @ronawho