This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

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

It's wrong to use std::system_category() for both GetLastError() and
errno on Windows.  Apparently, everybody uses std::generic_category()
for errno values, which appears to be a safe choice.

Some discussion on this confusing topic can be found here:

 https://stackoverflow.com/questions/28746372/system-error-categories-and-standard-system-error-codes

The Windows standard library emulates part of POSIX, which includes
errno.

Apparently, the std::system_error constructor appends strerror()
already.

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.

Simplify the definition of many build options.

Make sure glibc gives us all features.