preserve timestamps when installing headers

Hi hackers,

I noticed that `make install` updates modification time for all
installed headers. This leads to recompilation of all dependent
objects, which is inconvenient for example when working on a
third-party extension. A way to solve this would be to pass
`INSTALL="install -p"` to `configure`, to make `install` preserve the
timestamp. After this, a new problem arises -- the
`src/include/Makefile` doesn't use `install` for all headers, but
instead uses `cp`. This patch adds `-p` switch to `cp` invocation in
these files, to make it preserve timestamps. Combined with the
aforementioned install flag, it allows a developer to hack on both
postgres and a third-party extension at the same time, without the
unneeded recompilation.

--
Alexander Kuzmenkov
Timescale