build: use -pthread when available

Our code currently tries to link -lpthread and adds stuff like -D_REENTRANT
based on the target platform.

GCC actually supports to just pass a -pthread compiler and linker flag that
will automatically enable threading and define the correct symbols for the
platform, so let's try to just use that as our first choice.

clang also supports -pthread but it must be used only as a compiler flag,
so we also take care of that scenario with this commit.
cocoa: fix native fullscreen

This fixes a couple of issues with the Cocoa `--native-fs` mode, primarily:

 - A ghost titlebar at the top of the screen in full screen:

   This was caused by the window constraining code kicking in during
   fullscreen. Simply returning the unconstrained rect from the constraining
   method fixes the problem.

 - Incorrect behavior when using the titlebar buttons to enter/exit
   fullscreen, as opposed to the OSD button.

   This was caused by mpv's internal fullscreen state going out of sync with
   the NSWindow's one. This was the case because `toggleFullScreen:`
   completely bypassed the normal event flow that mpv expects.

Signed-off-by: Ryan Goulden <percontation@gmail.com>

Change style for mpv, simplify and refactor some of the constraining code.

Signed-off-by: Stefano Pigozzi <stefano.pigozzi@gmail.com>
cocoa: cosmetics: use NSMakeSize
cocoa: move sync section assignments in vo_control
cocoa: Use default NSWindow constraining

With recent work in place, it now has good behavior and custom constraining is unnecessary.