Changes to support MATRIX execution

[liorko87]

Hi,

In order to support MATRIX The following changes was done:

• Support for gcc < 8
• Remove relative paths from .gitmodules
• Add MATRIX directory

[lenerd]

Hi,
I noticed that in 11c4986 you sometimes used the macro _GNUC_ and on other times __GNUC_. The GCC documentation, however, defines __GNUC__ (double underscores on both sides).

Also, I guess this may break build with clang (when experimental header/symbols are removed), since they define __GNUC__ to the value 4.

[liorko87]

As for the macro, I suggest that we follow the documentation and use __GNUC__.
As for clang, maybe we can check

#ifdef __GNUC__
     #ifndef __clang__

[lenerd]

I close this in favor of #94.