Avoid direct func pointers to built-ins on MSVC
Avosd func pointers by default on all platforms

The cost of this change is minimal (e.g. on x64, ~100 bytes) but it's
something that would quite easily crop up on exotic platforms.  Fixing
it without tinkering with Duktape internals would be difficult for the
calling code.
Release note update
Merge branch 'wrapped-math-funcptrs'

Related to GH-17.