Handle CPATH variable being unset

Author: bjodah

.ci/_source_common_env.sh

@@ -1,6 +1,6 @@
 export CACHE_ROOT=$(pwd)/cache-ci
 export PYTHONUSERBASE=$CACHE_ROOT/pyusrb
-export CPATH=$SUNDIALS_ROOT/include:/usr/include/suitesparse:$CPATH  # sunlinsol_klu.h includes "klu.h"
+export CPATH=$SUNDIALS_ROOT/include:/usr/include/suitesparse:${CPATH:-}  # sunlinsol_klu.h includes "klu.h"
 export CPLUS_INCLUDE_PATH=${Boost_ROOT}/include
 export LIBRARY_PATH=$SUNDIALS_ROOT/lib
 export LD_LIBRARY_PATH=$SUNDIALS_ROOT/lib