Segv when building with spack toolchain

On my ubuntu machine:
```
spack install --keep-stage py-basalt@develop
gdb /usr/bin/python3 /path/to/spack-stages/spack-stage-py-basalt-0.2.8-n72ktmkhtcndrlaheglka6owknsbhbov/spack-src/core.python3.6.19170
GNU gdb (Ubuntu 8.1-0ubuntu3.2) 8.1.0.20180409-git
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Reading symbols from /usr/bin/python3...(no debugging symbols found)...done.
[New LWP 19170]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Core was generated by `/usr/bin/python3.6 -s setup.py --no-user-cfg build test'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007ff6f72bb5f7 in std::default_delete::operator() (this=, __ptr=) at /usr/include/c++/8/bits/unique_ptr.h:270
270 ~unique_ptr() noexcept
(gdb) bt
#0 0x00007ff6f72bb5f7 in std::default_delete::operator() (this=, __ptr=) at /usr/include/c++/8/bits/unique_ptr.h:270
#1 std::unique_ptr >::~unique_ptr (this=, __in_chrg=) at /usr/include/c++/8/bits/unique_ptr.h:274
#2 basalt::create_columns_families(nlohmann::basic_json, std::allocator >, bool, long, unsigned long, double, std::allocator, nlohmann::adl_serializer$
const&, std::__cxx11::basic_string, std::allocator > const&, rocksdb::Options&, std::shared_ptr) ()
at /goinfre/src/github.com/BlueBrain/spack/var/spack/stag/tcarel/spack-stage-py-basalt-0.2.8-n72ktmkhtcndrlaheglka6owknsbhbov/spack-src/src/basalt/config.cpp:305
#3 0x00007ff6f72bbabb in basalt::Config::configure(rocksdb::Options&, std::__cxx11::basic_string, std::allocator > const&) const () at /usr/include/c++/8/ext/atomicity.h:96
#4 0x00007ff6f72f0625 in basalt::GraphImpl<(basalt::EdgeOrientation)0>::GraphImpl (this=0x15e6c00, path="/tmp/tmpbpr5gy_0", config=..., throw_if_exists=) at /usr/include/c++/8/bits/unique_ptr.h:326
#5 0x00007ff6f72f0fe9 in basalt::GraphImpl<(basalt::EdgeOrientation)0>::GraphImpl (this=0x15e6c00, path="/tmp/tmpbpr5gy_0")
at /goinfre/src/github.com/BlueBrain/spack/var/spack/stag/tcarel/spack-stage-py-basalt-0.2.8-n72ktmkhtcndrlaheglka6owknsbhbov/spack-src/src/basalt/graph_impl.cpp:49
#6 0x00007ff6f72d1ca2 in basalt::Graph<(basalt::EdgeOrientation)0>::Graph (this=0x104a6e0, path="/tmp/tmpbpr5gy_0")
at /goinfre/src/github.com/BlueBrain/spack/var/spack/stag/tcarel/spack-stage-py-basalt-0.2.8-n72ktmkhtcndrlaheglka6owknsbhbov/spack-src/src/basalt/graph.cpp:26
#7 0x00007ff6f72a4bc7 in void pybind11::cpp_function::initialize, std::allocator > const&>::execute>, pybind11::arg, char const*, 0>(pybind11::class_>&, pybind11::arg const&, char const* const&)::{lambda(pybind11::detail::value_and_holder&, std::__cxx11::basic_str$
ng, std::allocator > const&)#1}, void, pybind11::detail::value_and_holder&, std::__cxx11::basic_string, std::allocator > const&, pybind11::name, pybind11::is_method, $
ybind11::sibling, pybind11::detail::is_new_style_constructor, pybind11::arg, char const*>(pybind11::class_>&&, void (*)(pybind11::detail::value_and_holder&, std::__cxx11::basic_string, std::allocator > const&), pybind11::name const&, pybind11::is_method const&, pybind11::sibling const&, pybind11::detail::is_new_style_constructor const&, pybind11::arg const&, char const* const&)::{lambda(pybind$
1::detail::function_call&)#3}::_FUN(pybind11::detail::function_call) [clone .lto_priv.547] ()
at /goinfre/src/github.com/BlueBrain/spack/var/spack/stag/tcarel/spack-stage-py-basalt-0.2.8-n72ktmkhtcndrlaheglka6owknsbhbov/spack-src/src/third_party/pybind11/include/pybind11/detail/init.h:61
#8 0x00007ff6f72af3c8 in pybind11::cpp_function::dispatcher(_object*, _object*, _object*) ()
at /goinfre/src/github.com/BlueBrain/spack/var/spack/stag/tcarel/spack-stage-py-basalt-0.2.8-n72ktmkhtcndrlaheglka6owknsbhbov/spack-src/src/third_party/pybind11/include/pybind11/pybind11.h:625
#9 0x00000000005674fc in _PyCFunction_FastCallDict ()
#10 0x0000000000595381 in ?? ()
#11 0x000000000054a8a5 in ?? ()
#12 0x0000000000551b81 in ?? ()
#13 0x00000000005aa6ec in _PyObject_FastCallKeywords ()
#14 0x000000000050abb3 in ?? ()
#15 0x000000000050c5b9 in _PyEval_EvalFrameDefault ()
#16 0x0000000000509d48 in ?? ()
#17 0x000000000050aa7d in ?? ()
#18 0x000000000050c5b9 in _PyEval_EvalFrameDefault ()
#19 0x0000000000508245 in ?? ()
#20 0x0000000000509642 in _PyFunction_FastCallDict ()
#21 0x0000000000595311 in ?? ()
#22 0x00000000005a067e in PyObject_Call ()
#23 0x000000000050d966 in _PyEval_Eval
```

It doesn't seem spack related.