Make sdists created on Linux compatible with other OSes

Cython doesn't compile 'IF SYSTEM_UNAME' checks into appropriate
C checks like '#ifdef __linux__'.  Instead it evaluates the constant
at the build time, meaning that

  IF SYSTEM_UNAME == 'Linux':
     cdef extern from "sys/epoll.h":
        ...

will be compiled to simple "include <sys/epoll.h>" (unguarded) on
Linux, meaning that the result C file won't compile on other OSes.

This commit moves the platform check to compat.h.

Author: 1st1

uvloop/handles/poll.pyx

@@ -61,7 +61,11 @@ cdef class UVPoll(UVHandle):
             self._fatal_error(exc, True)
             return
 
-        IF UNAME_SYSNAME == "Linux":
+        cdef:
+            int backend_id
+            system.epoll_event dummy_event
+
+        if system.PLATFORM_IS_LINUX:
             # libuv doesn't remove the FD from epoll immediately
             # after uv_poll_stop or uv_poll_close, causing hard
             # to debug issue with dup-ed file descriptors causing
@@ -71,10 +75,6 @@ cdef class UVPoll(UVHandle):
             # It's safe though to manually call epoll_ctl here,
             # after calling uv_poll_stop.
 
-            cdef:
-                int backend_id
-                system.epoll_event dummy_event
-
             backend_id = uv.uv_backend_fd(self._loop.uvloop)
             if backend_id != -1:
                 memset(&dummy_event, 0, sizeof(dummy_event))

uvloop/includes/compat.h

@@ -16,3 +16,14 @@
 #else
 #define PLATFORM_IS_APPLE 0
 #endif
+
+
+#ifdef __linux__
+#  define PLATFORM_IS_LINUX 1
+#  include <sys/epoll.h>
+#else
+#  define PLATFORM_IS_LINUX 0
+#  define EPOLL_CTL_DEL 2
+struct epoll_event {};
+int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event) {};
+#endif

uvloop/includes/system.pxd

@@ -62,16 +62,13 @@ cdef extern from "pthread.h" nogil:
 cdef extern from "includes/compat.h" nogil:
 
     cdef int EWOULDBLOCK
-    cdef int PLATFORM_IS_APPLE
-
 
-IF UNAME_SYSNAME == "Linux":
-
-    cdef extern from "sys/epoll.h" nogil:
+    cdef int PLATFORM_IS_APPLE
+    cdef int PLATFORM_IS_LINUX
 
-        struct epoll_event:
-            # We don't use the fields
-            pass
+    struct epoll_event:
+        # We don't use the fields
+        pass
 
-        int EPOLL_CTL_DEL
-        int epoll_ctl(int epfd, int op, int fd, epoll_event *event)
+    int EPOLL_CTL_DEL
+    int epoll_ctl(int epfd, int op, int fd, epoll_event *event)