configure.ac: check for highest available C++17/C++14/C++11 standard

configure.ac: check for highest available C++17/C++14/C++11 standard

There are external libraries (boost, glibmm) as well as platforms (Mac
OSX 12) which "require" C++ language standards above 11. Detect the
highest available standard which is supported by the compiler.

Use a "have modern C++" variable to improve robustness and readability
of the C++ language standard check.