[PATCH] Change Windows build docs to point to flex and bison from msys

Hi all

For some time it's been impossible to build PostgreSQL on 64-bit Windows
by following the documentation's advice, as the version of Flex we
distribute on the PostgreSQL FTP site does not work on 64-bit Windows
hosts. See this 2011 message (
http://archives.postgresql.org/pgsql-hackers/2011-07/msg00100.php).

The attached patch changes the documentation to point users to the
working flex and bison provided by msys - either as part of MinGW, or
from msysgit. It also mentions the error that people will get when
trying to use the old flex we distribute on 64-bit hosts so it's easier
to find out about the issue.

-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services