Allow using wxGraphicsContext with MinGW if gdiplus.h is available.

MinGW doesn't include gdiplus.h but it can be installed independently by the
user. Allow to use it if it's available:

- Include windows.h when checking for it in configure as this is apparently
  needed for its compilation.
- Don't reset wxUSE_GRAPHICS_CONTEXT to 0 for non-Microsoft compilers, instead
  just define it as 0 by default for them, allowing the user to simply change
  the definition in wx/msw/setup.h.

Closes #11892.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63832 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/chkconf.h

@@ -239,15 +239,6 @@
    Compiler-specific checks.
  */
 
-// Only MSVC 7+ has gdiplus.h, add exceptions for other compilers here if they
-// can support it too (but notice that Borland currently dies in
-// src/msw/gdiplus.cpp with "macro expansion too long" error even if the header
-// is available)
-#if !wxCHECK_VISUALC_VERSION(7)
-    #undef wxUSE_GRAPHICS_CONTEXT
-    #define wxUSE_GRAPHICS_CONTEXT 0
-#endif
-
 // Borland
 #ifdef __BORLANDC__