https://github.com/canonical/lightdm/pull/261

# A little bit modified version of Alpine linux patch [1]. On musl the default
# implementation of updwtmpx will give conflicting types error and build will
# fail. This patch should fix that. This issue is noticed together with bug
# 766866 [2].
# [1]: https://git.alpinelinux.org/aports/tree/community/lightdm/musl-updwtmpx.patch
# [2]: https://bugs.gentoo.org/766866
diff --git a/src/session-child.c b/src/session-child.c
index eef51e4..0052904 100644
--- a/src/session-child.c
+++ b/src/session-child.c
@@ -193,8 +193,13 @@ read_xauth (void)
 }

 /* GNU provides this but we can't rely on that so let's make our own version */
+#if defined(__GLIBC__)
 static void
 updwtmpx (const gchar *wtmp_file, struct utmpx *ut)
+#else // use this version in libc's other than glibc
+void
+updwtmpx (const char *wtmp_file, const struct utmpx *ut)
+#endif
 {
     struct utmp u;
     memset (&u, 0, sizeof (u));