Build: stop updating timestamp of st.h when not needed

Build: stop updating timestamp of st.h when not needed

Use the same approach as other generated headers (a temporary file,
compare to the existing, then copy), to avoid touching st.h, so
that other dependent objects are not rebuilt, if not needed.
It should speed up building when switching git branches, as often
config.status or automake are run, causing Makefiles to be recreated.

https://bugzilla.gnome.org/show_bug.cgi?id=638453