cmake: skip probes for required C++ standard

[tianrking]

Summary

• Skip the C++17/C++20 compiler feature probes when the consuming project has explicitly selected and required a C++ language standard.
• Preserve the existing probe path when no standard is selected, or when CMake is allowed to fall back to a lower standard.

Root cause

CMake/AbseilDll.cmake always ran two check_cxx_source_compiles calls, even when CMAKE_CXX_STANDARD and CMAKE_CXX_STANDARD_REQUIRED already establish the language-mode contract for every target. This adds configuration-time compilations without changing the selected feature level.

The new branch only derives the internal feature flags from CMAKE_CXX_STANDARD when the requested standard is required. Optional standards retain compiler probing so a CMake fallback cannot be mistaken for a guaranteed language mode.

Fixes #2023.

Validation

• Configured with -DCMAKE_CXX_STANDARD=17 -DCMAKE_CXX_STANDARD_REQUIRED=ON: 0 Abseil standard probe lines.
• Configured with -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=ON: 0 Abseil standard probe lines.
• Configured with -DCMAKE_CXX_STANDARD=20 -DCMAKE_CXX_STANDARD_REQUIRED=OFF: existing probes still run.
• Built the strings CMake target with both required C++17 and required C++20 configurations on MSVC.
• Ran git diff --check.