pgsql: Copy the libpq DLL to the bin directory on Mingw and Cygwin.

Copy the libpq DLL to the bin directory on Mingw and Cygwin.

This has long been done by the MSVC build system, and has caused
confusion in the past when programs like psql have failed to start
because they can't find the DLL. If it's in the same directory as it now
will be they will find it.

Backpatch to all live branches.

Branch
------
REL9_1_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/dfb4a1a21cb71f22ca49c4870cc7cfdc2d55f17b

Modified Files
--------------
src/interfaces/libpq/Makefile |    9 +++++++++
1 file changed, 9 insertions(+)