Set env into C-API library.

[gongweibao]

Reference: #4813 (comment) and #5280 (comment)

Now, we need to set env variables MKL_NUM_THREADS and OMP_NUM_THREADS before running C-API program to make the best performance of MKLML.

How to merge these into C-API library and so user don't concern about it?

[luotao1]

Can you update https://github.com/gongweibao/testcapi and tell us how to use paddle_capi_shared for build.sh at first?

/opt/compiler/gcc-4.8.2/sbin/../lib/gcc/x86_64-baidu-linux-gnu/4.8.2/../../../../x86_64-baidu-linux-gnu/bin/ld: cannot find -lpaddle_capi_shared
collect2: error: ld returned 1 exit status
make[2]: *** [exeModel] Error 1
make[1]: *** [CMakeFiles/exeModel.dir/all] Error 2
make: *** [all] Error 2