fix: Fix broken GCC 16 build (enforce C11/C++17 compile )

[cphlipot]

Summary

Currently the project requests the C11 and C++17 feature sets using
target_compile_features, however this only gaurentees us a minimum
C++ version of C++17, and allows compile to proceed with C++20
(the GCC 16 default) if it is already set.

Building Stable-diffusion.cpp with C++20 results in build failures
as std::u8string can no longer be assigned to std::string due to
C++20 standards changes.

Explicitly set the required language standard to C++17 in order to
ensure we pass/fail the build consistently across compiler toolchains
even if compilers by default try to expose a superset of C++17
features.

This fix copies the approach used by https://github.com/ggml-org/ggml/blob/master/CMakeLists.txt#L284-L288

A side effect of this change is that is also now enforces the CONTRIBUTING.md's requirement
to use C++17 at the CMake level.

Related Issue / Discussion

#1475

Additional Information

Tested build now succeeds on Fedora 44 with GCC 16 using the
following commands for vulkan build from build.md

Verify GCC 16+ is being used:

gcc --version

gcc (GCC) 16.1.1 20260501 (Red Hat 16.1.1-1)

then verify build:

mkdir build && cd build
cmake .. -DSD_VULKAN=ON
cmake --build . --config Release

Checklist

• I have read and confirmed this PR follows the contribution guidelines.

[candrews]

Tested this patch and it fixed the compilation problem for me.

$ gcc --version
gcc (Gentoo 16.1.0 p1) 16.1.0

[Nindaleth]

Thanks for unblocking the GCC 16 build!

[cphlipot]

Updated the PR. Now the CmakeLists.txt should enforce C11/C++17 at the project level instead of target level, aligning itself with how other projects like GGML do this. this also simplified the change a bit.

Also i noticed @leejet just added a CONTRIBUTING.md and PR template, so updated the PR to comply with those.