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.

(cherry picked from commit 0e6fe307c10d47efee3fb95c50aee9c0f01bc8ec)