build: avoid custom handling for C++ standard

Use the builtin CMake support for specifying the proper flags for the targets to
build at a certain C++ standard.  This avoids unnecessary checks in CMake,
speeding up the configure phase as well as simplifies the logic overall.

GitOrigin-RevId: 5ce2c6d2db88019e0b8bc00c77e4155b4eaa15d7