GCC 14 Adds Initial "-std=c++26" Plumbing

Similar to LLVM Clang 17 adding -std=c++26 support as the open-source compiler begins work on the next major revision of the C++ programming language, the GCC 14 compiler code has now also added the -std=c++26 compiler option.

With C++26 papers beginning to be approved and C++23 essentially finished, Red Hat's Marek Polacek has added the initial plumbing for -std=c++26 and the GNU dialect with -std=gnu++26 as well as also honoring c++2c and gnu++2c as alternative values.

This work establishes the C++2026 standard and just gets the infrastructure in place but doesn't yet begin adding any new C++26 features.

By the time of the GCC 14 release next year it's likely some experimental C++26 features will be added to the code-base, but for now at least as of this merged patch on Friday, the C++26 options are being recognized by the latest open-source compiler code.