bootstrap.sh fails to compile with clang/6.0.0 on quartz

While clang/6.0.0 fails, switching to gcc works fine.

Steps to reproduce:

1. ssh quartz
2. ml clang/6.0.0
3. git clone git@github.com:LLNL/scr.git && cd scr
4. ./bootstrap.sh

This results in the following build failure:

[ 77%] Building C object src/CMakeFiles/redset_o.dir/redset_reedsolomon_common.c.o
/usr/workspace/martymcf/src/scr/deps/redset/src/redset_reedsolomon_common.c:13:24: fatal error: kvtree_mpi.h: No such file or directory
#include "kvtree_mpi.h"
^
/usr/workspace/martymcf/src/scr/deps/redset/src/redset_reedsolomon.c:13:24: fatal error: kvtree_mpi.h: No such file or directory
#include "kvtree_mpi.h"
^
compilation terminated.

Looking through the output, it appears that MPI_C is found, but MPI_CXX is not.

-- Found MPI_C: /usr/tce/packages/mvapich2/mvapich2-2.3-clang-6.0.0/lib/libmpi.so (found version "3.1")
-- Could NOT find MPI_CXX (missing: MPI_CXX_WORKS)
-- Could NOT find MPI (missing: MPI_CXX_FOUND) (found version "3.1")