w32: Fix PKG_CONFIG_LIBDIR in --build-w32

* autogen.sh: Point pkg-config to the right location.

PKG_CONFIG_LIBDIR is located to usually be /usr/lib/pkgconfig so
in our case it should also point directly to the pkgconfig location
and not the prefix. This fixes gnutls and sqlite detection.

• Signed-off-by: Andre Heinecke <aheinecke@intevation.de>