CMakeLists.txt: always use highest available C++17/C++14/C++11 standard

CMakeLists.txt: always use highest available C++17/C++14/C++11 standard

Check for the availability of the C++17, C++14, and C++11 language
standards. Prefer the highest available to build any of the feature
tests or applications. Factor out common conditions into a central spot
in the CMake build rules. Rename variables to avoid special chars in
their name.

Setup both the CMAKE_CXX_STANDARD cmake variable which internally is
used transparently, as well explicitly pass the -std= compile flag in
build instructions for feature checks and application code. It's what
the smuview build does, should also work for pulseview, and is assumed
to not harm either in case it's redundant.

This unbreaks operation in most generic ways on platforms like MacOS 12
(system library), and for external libraries of differing degrees of
aggressivness (boost, glibmm, sig++).