Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

We're using std::chrono::steady_clock now.  No need to duplicate code.

This is Darwin specific: the previous implementation was causing an integer
overflow when base.numer is very large. On PPC Darwin, the timebase info is 1000000000/33330116 and this is too large for integer arithmetic.

This is Darwin specific: the previous implementation was causing an integer
overflow when base.numer is very large. On PPC Darwin, the timebase info is 1000000000/33330116 and this is too large for integer arithmetic.

All parameters seem mandatory, otherwise this call makes MPD crash.

Reduce GLib usage.

Don't use GTimer if the operating system is able to tell us the
uptime.