Harmonise the two methods to generate dependency files

One of them didn't clean away .d.tmp files properly.
The other would overwrite the .d files unconditionally, thereby
causing a possibly unnecessary dependency rebuild, which touches the
date of Makefile, which causes a possibly unnecessary rebuild of
buildinf.h and everything that depends on that.

Reviewed-by: Andy Polyakov <appro@openssl.org>
1 file changed