When you have a C/C++ project created without Intel SGX, you cannot use Intel SGX support. In this case, you need to add SGX nature to this project to use Intel SGX support:
Select Software Guard Extension Tools → Add SGX Nature
Add SGX Nature
After you add the SGX nature to your project, you should see:
A subdirectory sgx
in the project which contains a Makefile file.
Makefile for Intel(R) SGX
The Intel SGX tools as shown in the following graphic:
Intel(R) SGX Tools
New configurations specific to SGX technology. You may see the configurations for the project by clicking to the down arrow of button usually found at the top of the Eclipse window:
Configurations Specific to Intel(R) SGX Technology