ADIOS 2.5.0: SSC also uses JSON

Created by: ax3l

I am currently seeing a spack build issue with ADIOS 2.5.0 on CI of the form:

from /tmp/travis/spack-stage/adios2-2.5.0-tk6drsnu

            qt3zexkihltbumzfzi2j4a6b/spack-src/source/adios2/core/IO.cpp:42:

  >> 679    /tmp/travis/spack-stage/adios2-2.5.0-tk6drsnuqt3zexkihltbumzfzi2j4a

            6b/spack-src/source/adios2/toolkit/format/dataman/DataManSerializer

            .h:22:29: fatal error: nlohmann/json.hpp: No such file or directory

     680     #include <nlohmann/json.hpp>

https://travis-ci.org/openPMD/openPMD-api/jobs/598258587

The problem is likely in CMake blocks such as https://github.com/ornladios/ADIOS2/blob/v2.5.0/source/adios2/CMakeLists.txt#L161-L170 that also include toolkit/format/dataman/DataManSerializer.cpp|tcc but do not link nlohmann_json as in Dataman, etc.

Spack spec: adios2@2.5.0%gcc@4.9.4+blosc build_type=Release +bzip2~dataman~endian_reverse~fortran~hdf5~mpi+pic+png~python+shared+ssc+sst+sz~zfp arch=linux-ubuntu14.04-haswell