compositor: add fallback strchrnul()

Android does not have this function.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
diff --git a/configure.ac b/configure.ac
index 7dbd9f9..ce97486 100644
--- a/configure.ac
+++ b/configure.ac
@@ -27,7 +27,7 @@
 
 AC_CHECK_HEADERS([execinfo.h])
 
-AC_CHECK_FUNCS([mkostemp])
+AC_CHECK_FUNCS([mkostemp strchrnul])
 
 PKG_CHECK_MODULES(COMPOSITOR,
 		  [wayland-server egl >= 7.10 glesv2 xkbcommon pixman-1])