Makefile: Build with -Werror=date-time if the compiler supports it

GCC 4.9 and newer have a new warning -Wdate-time, which warns on any use
of __DATE__, __TIME__, or __TIMESTAMP__, which would make the build
non-deterministic.  Now that the kernel does not use any of those
macros, turn on -Werror=date-time if available, to keep it that way.

The kernel already (optionally) records this information at build time
in a single place; other kernel code should not duplicate that.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>