[openal] Recent changes for OpenAL Soft

[Chris Robinson]

It's been a while, but here's an update for you all.

The conversion to C++ is now largely done. The base standard is C++11, 
which in turn requires GCC 5*, Clang 3.6 (I think, maybe 3.8?), MSVC 
2014, or newer. Windows and Linux builds have been verified to work, and 
builds have been successfully made for OSX and Android (using Clang and 
libc++).

* Technically, GCC started supporting C++11 in 4.8 and 4.9. However, 
those versions are missing some details from the standard that only got 
added in the 5.0 branch, which the codebase trips over.

In addition, SOFA support was added to makehrtf via libmysofa 
<https://github.com/hoene/libmysofa>, so you can make .mhr files for 
HRTF from .sofa files. It's available from at least some Linux distros' 
repos (Debian Stretch for sure), and didn't seem to be too hard to build 
on Windows. This is now required to build the utility, along with the 
companion sofa-info utility (used to help create def files for 
makehrtf). If the library isn't available, those utilities won't be 
built. It is not needed for the OpenAL Soft library itself, either 
building or running.


That's essentially it for now, regarding updated build requirements. If 
there's something I forgot to mention and you're hitting a snag with 
building master, please let me know and/or file an issue over on GitHub. 
The task of splitting the library up to audit/rewrite the code and 
relicense it is still on the table, and will get done in due time.