[Patch] Mingw: Fix import library extension, build actual static libraries

Currently building postgresql for Win32 with a mingw toolchain produces
import libraries with *.a extension, whereas the extension should be
*.dll.a. There are various downstream workarounds for this, see i.e. [1]
and [2]. The attached patch 0001-Fix-import-library-extension.patch
addresses this.

Related, no actual static libraries are produced alongside the
respective dlls. The attached patch 0002-Build-static-libraries.patch
addresses this, in a similar fashion as is already done for the AIX case
in Makefile.shlib.

Thanks Sandro

[1]
https://src.fedoraproject.org/rpms/mingw-postgresql/blob/master/f/mingw-postgresql.spec#_144
[2]
https://aur.archlinux.org/cgit/aur.git/tree/0001-Use-.dll.a-as-extension-for-import-libraries.patch?h=mingw-w64-postgresql