cms-common: fix MANPATH for C Shells

We need to check if MANPATH is set or not. Otherwise setenv
will fail with undefined variable.

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>
Merge pull request #124 from davidlt/IB/CMSSW_7_0_X/stable

[CMSSW_7_0_X] Check MANPATH env variable before setting in C Shell