wayland: Add a sigtimedwait() implementation for OpenBSD

sigtimedwait() is an optional part of POSIX.1-2001, and OpenBSD doesn't implement it. Add a replacement implementation based on https://comp.unix.programmer.narkive.com/rEDH0sPT/sigtimedwait-implementation

(backported from commit 1049426a76)
(cherry picked from commit d31d98d436)

CMakeLists.txt

@@ -1099,6 +1099,7 @@ if(SDL_LIBC)
     endforeach()
 
     check_symbol_exists(sigaction "signal.h" HAVE_SIGACTION)
+    check_symbol_exists(sigtimedwait "signal.h" HAVE_SIGTIMEDWAIT)
     check_symbol_exists(setjmp "setjmp.h" HAVE_SETJMP)
     check_symbol_exists(nanosleep "time.h" HAVE_NANOSLEEP)
     check_symbol_exists(sysconf "unistd.h" HAVE_SYSCONF)