Added separate import lib for msw .dlls

Added separate import lib for msw .dlls
Removed useless mingw and cygwin options.
Fixed library naming for msw _gl and set 'unix' style names for cross
builds and 'windows' style for native builds.
Added $target to wx-config, and fixed lib/header dir exclusions.
Added -mwindows to wx-config output for gui libs.
Fixed wx-config to output import lib instead of dll for shared
msw builds.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@12284 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775