pgsql: Use system install program when available and usable

Use system install program when available and usable

In a3176dac22c4cd14971e35119e245abee7649cb9 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.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/9db7ccae2000524b72a4052352cbb5407fb53b02

Modified Files
--------------
configure              |  109 ++++++++++++++++++++++++++++++++++++++++++++++++
configure.in           |    8 ++++
src/Makefile.global.in |    8 +++-
3 files changed, 123 insertions(+), 2 deletions(-)