Use system install program when available and usable

In a3176dac we switched to using
install-sh unconditionally, because the configure check
AC_PROG_INSTALL would pick up any random program named install, which
has caused failure reports
(http://archives.postgresql.org/pgsql-hackers/2001-03/msg00312.php).
Now the configure check is much improved and should avoid false
positives.  It has also been shown that using a system install program
can significantly reduce "make install" times, so it's worth trying.