Use dependencies for 'make headers_install'.

Re-export header files only if either they or their controlling Kbuild
file has actually changed. Also allow for similar dependencies with
'headers_check', once we properly create the dependencies for those.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

diff --cc Makefile
Simple merge

diff --cc scripts/Makefile.headersinst
Simple merge

diff --cc scripts/hdrcheck.sh
Simple merge