cmake: set C standard to GNU C due to inline assembly usage

[magiruuvelvet]

Standard ISO C does not have inline assembly support, but is a compiler extension. For compilers which default to ISO C, specify the use of GNU extensions to enable inline assembly.

Fixes compiler errors like this:

error: use of undeclared identifier 'asm'

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I'm opening this pull request as a package maintainer, rather than a user of this library. Please feel free to ignore this pull request if it is not fitting for this project. I just want to bring attention to this issue and hopefully safe someone else a couple of headaches. I'm not even sure if this is the proper location to add this flag.

For reference: I'm using clang with extensions disabled by default, when -std= is not specified on the command line.

EDIT: I should mention that this is a very niche use case, since you must actively change the C standard (when -std= is omitted from the command line) during the configure step when building your own copy of LLVM. No sane compiler distribution will be configured like this.

[wtdcode]

Thanks for the issue. Which package are you maintaining?

[magiruuvelvet]

Local packages for myself to run on my Steam Deck. I recently picked up Vita3K which makes use of unicorn. yuzu-emu also used unicorn in the past (removed by now), where I had the exact same problem during compilation.

[wtdcode]

This obviously conflicts with our:

set(CMAKE_C_STANDARD 11)

[mlgiraud]

The documentation says this:

When compiling in ISO C mode by GCC or Clang (e.g. with option -std=c11), __asm__ must be used instead of asm.

So replacing all asm usages by __asm__ should fix this i presume.

[magiruuvelvet]

I compiled the version in the master branch just fine with my toolchain. Downstream is simply outdated or may contain custom patches which break the passing of the -std=gnu... flag, so I guess this issue is invalid for this repository.

For that, I will just continue to provide my own patches for the packaging process. It's not that much of a hassle for me.

I found the CMAKE_C_EXTENSIONS variable in the CMake documentation, but it is supposed to be ON by default. I don't have custom patches integrated into my CMake which would overwrite the behavior of this variable.

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Regarding the __asm__ comment. This snippet of code compiled just fine in ISO C mode.

int main() {
    __asm__("nop");
    return 0;
}