Fix missing bx headers when compiling SHARED
Fix compat and tinystl being installed in include root
Move submodule specific installs to their respecive config files.
The custom command didn't recognize bgfx::shaderc and the generator expression seems bettter. I also added an include option since it might be important to be able to define those.
Added changes to each of the cmake files to make sure bgfx:: is defined
Also add an output variable name parameter.
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