The value of CMAKE_CXX_STANDARD should be encoded in installed file names to allow packaging for users requiring different -std=c++NN

Since different depending projects require different -std=c++NN settings in abseil, it should be possible to have abseil simultaneously installed with different -std=c++NN settings.

Google's or-tools require ```-std=c++17```, while others require ```-std=c++11``` and fail to build otherwise.

Please change the installed file paths to reflect ```-std=c++NN``` set through ```CMAKE_CXX_STANDARD```.
For example, ```include/absl/``` could become ```include/absl-c++NN/```, and ```lib/libabsl_XX.so``` could become ```lib/libabsl-c++NN_XX.so```.

Original bug: https://github.com/abseil/abseil-cpp/issues/819#issuecomment-709529075