BLD: enforce C++11 std for boost + xl_r (#13829)
BLD: enforce C++11 std for boost + xl_r (#13829)

* BLD: enforce C++11 std for boost + xl_r

* the spack `cxxstd` variant is not sufficient to enforce
`-std=c++11` usage in boost compile lines when `xl_r` compiler
spec is in use; while it would be nice if this were fixed
in a boost config file somewhere, for now this patch
allows boost to build on POWER9 with
an %xl_r compiler spec if the user specifies i.e.,:

`spack install boost@1.70.0+mpi cxxstd=11 %xl_r@16.1.1.5`

* Update var/spack/repos/builtin/packages/boost/package.py

Co-Authored-By: Adam J. Stewart <023e75a89b1f5ca1e59909f8756c185885836ad9@gmail.com>

iulian787/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack