Update CMakeLists.txt (fixes #102)

[pdimov]

• Adds a GHA job for running the tests "standalone" (against a CMake-installed Boost)
• Regenerates CMakeLists.txt using boostdep --cmake sort and then reapllies the standalone changes, but only when the library is the root project
• Replaces setting the CMAKE_CXX_STANDARD unconditionally to 11 (which may override a root project or user choice of e.g. 17) with target_compile_features