Unbundle build dependencies

[mnsanghvi]

Currently build dependencies like stb and miniaudio are bundled with the source code as git sub-modules. Using system provided versions of the dependencies would be a good idea. This would allow for consistency in builds as on any given OS version, we're always building against the same version of the dependency, as provided by the OS package management system.

This is a request to

• remove the dependency from the source distribution
• build against the OS/distribution provided version of the dependencies
• document the new dependency requirements and how to install them on different OS/distributions

The impetus for this came from the work done to get SmallBASIC building on FreeBSD and being included in the FreeBSD ports system as documented in #217