lib: Use PROJECT_SYSTEM to provide compile time warning

[glneo]

Several of these were not correct in some spots. Use the automatic replacement to fill this in when installing the headers.

[arnopo]

I would prefer to limit the use of cmake variable in source code when possible.

Could you detail side effect you observe?

[glneo]

What happens here for example is if I #include <metal/system/freertos/alloc.h>, the compiler errors out with: Include metal/alloc.h instead of metal/freertos/alloc.h. Which would be more helpful if I had included metal/freertos/alloc.h anywhere, I could grep that and fix it, but the included file is metal/system/freertos/alloc.h.

Maybe an easier solution is just to have the message state: "Do not include this file directly, include <metal/alloc.h> instead". Then folks could just see what file they included from the compiler error line and remove that inclusion.

[edmooring]

I like your second solution avoiding the CMake variables in the code better.

[arnopo]

Maybe an easier solution is just to have the message state: "Do not include this file directly, include <metal/alloc.h> instead". Then folks could just see what file they included from the compiler error line and remove that inclusion.

+1 for this solution