Fix warnings

[bbonev]

No description provided.

[lu-zero]

It might be more portable to use a #define and set it only if gcc or clang are in use.

[bbonev]

An autoconf test plus a couple of ifdefs based on the result...

I did it that way because I think that ancient compilers should silently ignore unsupported attributes.

Do you have an example ancient compiler to test with? Then we can adjust to the resulting build with it.

[ArsenArsen]

(void)unused_arg; is also portable.

[bbonev]

Now I see that there is one change that deserves a separate PR and review and it got included here (my mistake): dce2732#diff-74ecd06b7b5ba73ee97dbca326e44e3dddf6e8841bda2e073b06bdc4d8bd158d - this one is more serious than the rest from this PR and requires extra attention

About the unused arguments warnings - eudev is Linux only and I have been thinking about a possible scenario where the __attribute__((unused)) may become a problem and I couldn't think of one...

[lu-zero]

About the unused arguments warnings - eudev is Linux only and I have been thinking about a possible scenario where the __attribute__((unused)) may become a problem and I couldn't think of one...

We can restrict to gcc and clang since pcc/tcc/cparser should fail already for unrelated reasons and swcc is no more. I guess just documenting it should be fine.

[bbonev]

I have added a note in the build instructions about the modern C compiler requirement.
Also did some rewording of the neighboring paragraphs to use more passive language...

@lu-zero Can you please provide build from source instructions for Gentoo?

[lu-zero]

Sure, I double checked and we already depend on modern (#pragma once) and gnu-compatible (__attribute__) compilers already.