cmake: Fix hardening flags

[hebasto]

During testing #93, a few issues were noticed:

• -mbranch-protection=bti shouldn't be in C flags

The other change simplifies the summary code in #93.

[theuni]

A few things:

1. Master currently adds -mbranch-protection=bti to ldflags. Agree that it shouldn't be, and this change looks correct, just pointing out that this isn't something introduced by CMake.
2. The meaning of core_base_interface is becoming quite fuzzy to me. What happens when we switch to the upstream CMake build for secp?
3. c-i error below:

CMake Error at CMakeLists.txt:499 (target_link_libraries):
  Error evaluating generator expression:

    $<COMPILE_LANGUAGE:CXX,OBJCXX>

  $<COMPILE_LANGUAGE:...> may only be used to specify include directories,
  compile definitions, compile options, and to evaluate components of the
  file(GENERATE) command.

[hebasto]

Dealing with an individual options within generator expressions is much easier.

[hebasto]

@theuni

A few things:

1. Master currently adds -mbranch-protection=bti to ldflags. Agree that it shouldn't be, and this change looks correct, just pointing out that this isn't something introduced by CMake.

I left only this change, which effectively is pulled from the current #93.

2. The meaning of core_base_interface is becoming quite fuzzy to me. What happens when we switch to the upstream CMake build for secp?

I'll submit a demo branch shortly.

[hebasto]

I'll submit a demo branch shortly.

@theuni

I've updated #192, which is currently based on #93. I hope it will answer your question and resolves your concerns regarding the meaning of core_base_interface.

[m3dwards]

ACK e2f2f23

Tested with and without PR and can now see the hardening flag not being passed during linking on M1 Mac.