Dynamically find correct installation docs in Makefile.

Dynamically find correct installation docs in Makefile.

The base Makefile will output help to the user when invoking make in
an unconfigured tree, the help was however always referring to a file
which may not be present as it's only in tarballs.  Dynamically check
for the presence of the INSTALL file and fall back on README.git when
it's not available (which is the case of Git checkouts).

Reported-by: Tim McNamara <tim@mcnamara.nz>
Reviewed-by: Magnus Hagander <magnus@hagander.net>
Reviewed-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/730dae39-abaa-4140-893b-95d732fed003@www.fastmail.com