Fix remaining benchmarks with GCC 15

[lukel97]

In GCC 15 the default C standard was changed to gnu23, so similar to #285 this explicitly sets -std=gnu17 on benchmarks that are known to fail to build with GCC 15 because of this.

Whilst we're here, this also goes through and replaces -std=X flags with the more CMake-native C_STANDARD property.

[asb]

From what I can see in the CMake documentation, C_STANDARD or CXX_STANDARD will expand to -std={c,gnuc}NN or std={c++,gnuc++}NN depending on C_EXTENSIONS or CXX_EXTENSIONS (both defaulting to true). So unless I'm mistaken, some of these changes will be moving from std to the gnu language options.

I considered suggesting always explicitly defining C_EXTENSIONS or CXX_EXTENSIONS so it's easier to tell at a glance whether the gnu or standard version is being used, but the naming of the CMake property is so vague I'm not sure it would add a lot of clarity!

[lukel97]

From what I can see in the CMake documentation, C_STANDARD or CXX_STANDARD will expand to -std={c,gnuc}NN or std={c++,gnuc++}NN depending on C_EXTENSIONS or CXX_EXTENSIONS (both defaulting to true). So unless I'm mistaken, some of these changes will be moving from std to the gnu language options.

From what I understand both Clang and GCC (pre 15.0) default to -std=gnu17/gnu++17 so they already have the gnu extensions enabled: https://clang.llvm.org/docs/UsersManual.html#id77

Edit: Sorry I see now that you're talking about the changes that replace the explicit -std=X flags, good point. I'll remove those from this PR and maybe open another PR to move those over seeing as it's not the same thing, thanks for catching that.