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>

tests/test-runner.c

@@ -308,6 +308,13 @@ is_debugger_attached(void)
 
 	return rc;
 }
+#else
+static int
+is_debugger_attached(void)
+{
+	/* 0=debugger can't be determined */
+	return 0;
+}
 #endif
 
 int main(int argc, char *argv[])