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
------
REL8_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/ae3c98b9b10659e9d9c1dfb375624b0e6a299b30

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