Enable C++17 by default when building against Qt 6

[J5lx]

Also cleans up other compiler options a bit:

• use strict_c/c++ QMake feature to disable compiler extensions instead of messing directly with compiler flags
• same with msvc_mp and utf8_source. the latter also does compiler detection so there should be no need for special handling of legacy builds
• -stdlib=libc++ for macOS is removed since it appears to be the default since Qt 5.1
• QMAKE_LINK{,_SHLIB} = $$QMAKE_CXX also appears to be the default

[J5lx]

macOS build failures are unrelated and will be fixed by #1823.