Create secure build of bgfx

[ikeycode]

We should rely as much as possible on system libraries from the build host, to avoid the situation we have with vendored code. For example, on Solus, we already have glslang, vulkan headers, freetype2 headers, etc. More of a "first release" target than an alpha one but important nonetheless.

[ikeycode]

Note that much of bgfx itself won't be linking to libs, except for opengl/etc. The tools we need to link to system libs, such as texturec, geometryc, etc. The bundled lib is pretty OK - just needs improvement.