Fix crashes to illegal instructions execution

[fabiosky]

Description of changes: Have the rust global build script build.rs generate the cargo config file.

The cargo config file will contain the intersection of:

• The instruction sets required by dcv-color-primitives (after a research it can be found it uses sse2, avx, avx2 and no more)
• The instruction sets really supported on the machine where the build happens

In this way, we prevent invalid instructions being executed on machines that do not support them.

I tested this on a RHEL6 machine (without avx support) and the generated cargo file looks the following:

[build]
rustflags = ["-g","-C","target-feature=+sse2"]

cargo test works fine.

Instead, on another machine with avx2 support the following config file is generated:

[build]
rustflags = ["-g","-C","target-feature=+sse2,+avx,+avx2"]

[fabiosky]

@nacho can you give a look at this? I also tried this patch on Windows and it is working fine.

[pbor]

This sounds weird... are we user build.rs gets executed before cargo itself reads the config?