BUILD: fix "make dist" failure on some linux distro with old csh/tcsh

on some linux distro (sles11sp2) csh fails to parse $LS_COLORS and borks with error:
Unknown colorls variable `mh'.

The workaround is to unset LS_COLORS before calling to csh script

reviewed by Jeff

cmr=v1.8:reviewer=ompi-rm1.8