JSON libraries need GCC >=4.9 or Clang >=3.4

[0ut0fcontrol]

my machine:

• CentOS 7
• gcc 4.8.5
• Clang 3.4.2
• cuda 9.2
• tensorflow 1.8

cmake use gcc as default, compling JSON libraries will get Error for old gcc version.

cd deepmd-kit/source/build
cmake3 -DXDRFILE_ROOT=$xdrfile_root -DTENSORFLOW_ROOT=$tensorflow_root -DCMAKE_INSTALL_PREFIX=$deepmd_root -DTF_GOOGLE_BIN=true ..
make

[ 88%] Building CXX object md/CMakeFiles/dp_mdnn.dir/mdnn.cc.o
In file included from /pubhome/jcyang/git/deepmd-kit/source/md/mdnn.cc:11:0:
/pubhome/jcyang/git/deepmd-kit/source/3rdparty/json.hpp:66:10: error: #error "unsupported GCC version - see https://github.com/nlohmann/json#supported-compilers"
#error "unsupported GCC version - see https://github.com/nlohmann/json#supported-compilers"
^
cc1plus: warning: unrecognized command line option "-Wno-ignored-attributes" [enabled by default]

If you have newer version clang (>=3.4), you can try to use clang, it work for me.
https://stackoverflow.com/questions/7031126/switching-between-gcc-and-clang-llvm-using-cmake