Re-add the glm interface library in cmake

[kiroma]

Also re-work the main cmakelists to make the project more easily embeddable

[Groovounet]

I am not getting the purpose of these changes... GLM doesn't need to be linked to be used.

What these interfaces library does?

[kiroma]

add_library(dummy INTERFACE)
target_include_directories(dummy INTERFACE my_include_dir)
add_executable(my_executable main.cpp)
target_link_libraries(my_executable dummy)

The INTERFACE keyword in target_include_directories means that when another target links against dummy, it must also have the includes specified for dummy, therefore it's about an equivalent of saying target_include_directories(my_executable PRIVATE my_include_dir)

In add_library it means that the target does not have any output.

This way the library author can specify which directories should be included by the user, while the user simply has to link against a dummy target which sets everything up.
include_directories is problematic as it gets completely omitted when the project is added via add_subdirectory.

[fuzzyTew]

@kiroma it doesn't look like people are successfully able to use this (see linked issue). Are you able to give a quick description of how embedding is expected to work now?

[kiroma]

Oh god I have not looked at this in forever. I thought someone else would pick it up.
A FindGLM.cmake module has to be written. I'll take a look and see what I can come up with.