dependencies fix, check for GLM with pkg-config first

[bartman]

On my system, Linux (NixOS), Cmake was unable to find GLM, which is accessible via GLM.
However, pkg-config works.

The patch provided, will try to use pkg-config, then find_package, before falling back to fetchcontent.

Hope that's acceptable.

[sudopluto]

you might want to just use the fetchcontent example in the glm readme:

https://github.com/g-truc/glm/blob/master/readme.md#cmake-using-fetchcontent

cmake_minimum_required(VERSION 3.11) # FetchContent is new in version 3.11.

include(FetchContent)

FetchContent_Declare(
	glm
	GIT_REPOSITORY	https://github.com/g-truc/glm.git
	GIT_TAG 	0af55ccecd98d4e5a8d1fad7de25ba429d60e863 #refs/tags/1.0.1
)

FetchContent_MakeAvailable(glm)

target_link_libraries(main PRIVATE glm::glm)

here is another ref from documentation:
https://github.com/g-truc/glm/blob/master/manual.md#-15-finding-glm-with-cmake

at least the way it seems, the only thing we want vendored by the OS is opengl (even that gets complicated on linux with glvnd & hybrid graphics)