Fix epoll_pwait2 feature detection

The Termux Android build can expose an epoll_pwait2 symbol while omitting the corresponding declaration from <sys/epoll.h>. The previous mkmf probe only checked whether the symbol could be linked, so extconf.rb generated HAVE_EPOLL_PWAIT2 even though epoll.c could not compile a real call to the function under C99 implicit-declaration rules.

Probe epoll_pwait2 using the same header and a concrete call expression instead. This keeps HAVE_EPOLL_PWAIT2 enabled on platforms where the header declares the API, while allowing Android/Termux to fall back to the existing epoll_wait path.

Closes #155

Author: samuel-williams-shopify

ext/extconf.rb

@@ -55,7 +55,7 @@
 have_func("&rb_process_status_wait")
 have_func("rb_fiber_current")
 have_func("&rb_fiber_raise")
-have_func("epoll_pwait2") if enable_config("epoll_pwait2", true)
+have_func("epoll_pwait2(0, 0, 0, 0, 0)", "sys/epoll.h") if enable_config("epoll_pwait2", true)
 
 have_header("ruby/io/buffer.h")