Exclude more logic when compiling with librashader=false

[Dragorn421]

Current configuration options wrt librashader

Building currently supports librashader=false (default true) to disable librashader support
Currently, it merely means not copying the library binary to the output (for the platforms that do)

This PR

This PR defines the NO_LIBRASHADER cpp macro under librashader=false, causing shader support in ares to be more thoroughly disabled, some code is #if'd out and the Shader menu in the UI is disabled.

Also featuring tiny bits of cleanup in related parts of the Makefile and code.

Comments are welcome. I'm far from a C++ expert.

Motivation

This was prompted to me by ares complaining in the console like

OpenGL: Failed to load librashader: shaders will be disabled

I personally don't use shaders so I tried compiling with librashader=false, which didn't get rid of that warning, so here we are. I could also just #if out the warning but it didn't feel as proper.