Use normal install program to install server headers.

Use normal install program to install server headers.

Commit a7032690f9 replaced $(INSTALL) with plain "cp" for installing the
server header files. It sped up "make install" significantly, because
the old logic called $(INSTALL) separately for every header file,
whereas plain "cp" could copy all the files in one command. However, we
have long since made it a requirement that $(INSTALL) can also install
multiple files in one command, see commit f1c5247563. Switch back to
$(INSTALL).

Discussion: https://www.postgresql.org/message-id/200503252305.j2PN52m23610%40candle.pha.pa.us
Discussion: https://www.postgresql.org/message-id/2415283.1641852217%40sss.pgh.pa.us