add basic support for cmake

[MrJake222]

A lot of projects (for ex. pico-sdk) use cmake as their compilation helper. I needed to include this in my Pico project, so here it is.

It becomes trivial to use (and also add compile flags). Example:

add_subdirectory(littlefs)
target_compile_definitions(littlefs PUBLIC LFS_THREADSAFE)

[geky-bot]

Tests passed ✓, Code: 16666 B (+0.0%), Stack: 1432 B (+0.0%), Structs: 788 B (+0.0%)

	Code	Stack	Structs		Coverage
Default	16666 B (+0.0%)	1432 B (+0.0%)	788 B (+0.0%)	Lines	2316/2496 lines (+0.0%)
Readonly	6126 B (+0.0%)	448 B (+0.0%)	788 B (+0.0%)	Branches	1184/1506 branches (-0.0%)
Threadsafe	17494 B (+0.0%)	1432 B (+0.0%)	796 B (+0.0%)		Benchmarks
Multiversion	16742 B (+0.0%)	1432 B (+0.0%)	792 B (+0.0%)	Readed	29369693876 B (+0.0%)
Migrate	18350 B (+0.0%)	1736 B (+0.0%)	792 B (+0.0%)	Proged	1482874766 B (+0.0%)
Error-asserts	17302 B (+0.0%)	1424 B (+0.0%)	788 B (+0.0%)	Erased	1568888832 B (+0.0%)

[geky]

Hi @MrJake222, thanks for creating a PR.

I appreciate the thought to upstream this, but I'm hesitant to bring this in unless more people weigh in that this is useful.

The problem is that there's always N+1 build systems, and my general experience is cmake configuration ends up very project-specific. Though those projects could have been using cmake "wrong".

---

We would probably also need to add cmake to the CI to at least test that the configuration continues to build correctly.

[zyglyf]

This would definately be usefull for us when integrtating littlefs into our CMake based projects. Having to add littlefs with ExternalProject_add() and setting a BUILD_COMMAND with a lot of parameters to get it into a CMake project is just not a good solution and is highly dependent of the format of the littlefs makefile.

[Oakchris1955]

Adding CMake support would be really helpful for those using CMake for their projects. On the other hand:

cmake configuration ends up very project-specific

I believe that it would be better just to create a section in the README describing which CMake commands to include in one's CMakeLists.txt in order to link the littlefs source files