MinGW: Link with shell32.dll instead of shfolder.dll.

This improves consistency with the MSVC build.  On buildfarm member
narwhal, since commit 846e91e0,
shfolder.dll:SHGetFolderPath() crashes when dblink calls it by way of
pqGetHomeDirectory().  Back-patch to 9.4, where that commit first
appeared.  How it caused this regression remains a mystery.  This is a
partial revert of commit 889f0381, which
adopted shfolder.dll for Windows NT 4.0 compatibility.  PostgreSQL 8.2
dropped support for that operating system.