shared: add file_create_dated() helper

For easy creation of unique new files. I'm looking at you,
screenshooter.

This code is based on timeline.c weston_timeline_do_open().

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
Reviewed-by: Daniel Stone <daniels@collabora.com>
diff --git a/Makefile.am b/Makefile.am
index 7fb9e17..4e219e1 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -884,6 +884,8 @@
 	shared/config-parser.c			\
 	shared/option-parser.c			\
 	shared/config-parser.h			\
+	shared/file-util.c			\
+	shared/file-util.h			\
 	shared/os-compatibility.c		\
 	shared/os-compatibility.h