By default, use more traditional linking pattern with meson.
The janet.exe will now link to janet-x.x.dll on windows (and
similar for linux/posix) when built with meson. This is slightly
less efficient and means that janet.exe built this way is no longer
standalone (you would need to move the dll along with the exe), but
plays better with most build systems.
This calls setvbuf to change FILE buffering. A goal is
to be able to use the existing file/* functions for blocking
IO analogous to `read` and `write` system calls.
One is a way to export symbols, the other a way to reference
API functions. Also include prebuilt dlljanet.dll and dlljanet.lib
for windows to save people the trouble of compiling janet.c themselves.
Instead of setting a flag, each interrupt increments an atomic
counter. When the interrupt is finally handled, either by scheduling
code to run on the event loop or executing some out of band code, the
user must now decrement the interrupt counter with
janet_interpreter_interrupt_handled. While this counter is non-zero, the
event loop will not enter the interpreter. This changes the API a bit but
makes it possible and easy to handle signals without race conditions
or scheduler hacks, as the runtime can ensure that high priority code is
run before re-entering possibly blocking interpreter code again.
Also included is a new function janet_schedule_soon, which prepends to
the task queue instead of appending, allowing interrupt handler to skip
ahead of all other scheduled fibers.
Lastly, also update meson default options to include the
interpreter_interrupt code and raise a runtime error if os/sigaction
is used with interpreter interrupt but that build option is not enabled.
