Changes to build IPP Crypto from source

[sbellem]

Please not that this is a draft PR and it needs some adjustments. I am putting it up so that it can benefit from feedback, and also it can be used as a patch in nix derivations that build IPP Crypto from source.

The goal of this PR is to allow building the IPP Crypto from source, and more particularly with nix. Moreover, the PR also aims to address issues encountered when building the SDK from source, with the IPP Crypto built-from-source as well.

• Issue sgx_ippcp.h: No such file or directory #719, add missing header file sgx_ippcp.h required when building the SDK from source
• Figure out a more general mechanism to build IPP Crypto from source (see comments below and commit message 7dd8f19)

The main thing that needs to be properly addressed is the usage of NIX_PATH and the check for the ipp submodule, which should not trigger the git submodule or git clone command when building from source with nix, as the ipp crypto submodule has already been fetched at the correct revision.

The work in progress commit 7dd8f19 provides more detailed information, copied here below for convenience:

WIP: figure more general mechanism to build ipp crypto

When building SDK & IPP Crypto from source with nix, the current check
for the IPP source code does not work properly. That is, even though the
IPP Crypto source code is there the check in the Makefile triggers the
call to the ipp_source target which invokes git to update the submodule
or clone the repo.

In the context of a nix derivation, the source code along with the
submodules is fetch at the beginning and the .git directory is removed,
such that later invocations to git commands such as the one in the
Makefile for the ipp_source target, fail.

Also, depending on the environment, NIX_PATH may not be set and is
therefore too restrictive as a check or assumes too much. A more general
mechanism would be preferable.