build: fix build and provide compat for OpenBSD

- wayland-egl-abi-check: try to use llvm-nm first instead of BSD nm (incompatible options)
- avoid forcing _POSIX_C_SOURCE=200809L (SOCK_CLOEXEC become available)
- epoll(7) is provided by a userspace wrapper around kqueue(2) as FreeBSD
- when using SO_PEERCRED, the struct to use is `struct sockpeercred` instead of `struct ucred` on OpenBSD
- provide a compatibility layer for count_open_fds() using sysctl(2) as FreeBSD

Signed-off-by: Sebastien Marie <semarie@online.fr>

src/wayland-os.c

@@ -111,7 +111,11 @@ int
 wl_os_socket_peercred(int sockfd, uid_t *uid, gid_t *gid, pid_t *pid)
 {
 	socklen_t len;
+#if defined(__OpenBSD__)
+	struct sockpeercred ucred;
+#else
 	struct ucred ucred;
+#endif
 
 	len = sizeof(ucred);
 	if (getsockopt(sockfd, SOL_SOCKET, SO_PEERCRED, &ucred, &len) < 0)