mapi/glapi: Fix dll linkage of GLES1 symbols.

This fixes several MSVC warnings like:

  warning C4273: 'glClearColorx' : inconsistent dll linkage

In fact, we should avoid using `declspec(dllexport)` altogether, and use
exclusively the .DEF instead, which gives more precise control of which
symbols must be exported, but all the public GL/GLES headers practically
force us to pick between `declspec(dllexport)` or
`declspec(dllimport)`.

Cc: "10.4" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/mapi/glapi/SConscript

@@ -16,6 +16,7 @@ if env['platform'] == 'windows':
     env.Append(CPPDEFINES = [
         '_GDI32_', # prevent gl* being declared __declspec(dllimport) in MS headers
         'BUILD_GL32', # declare gl* as __declspec(dllexport) in Mesa headers
+        'KHRONOS_DLL_EXPORTS', # declare gl* as __declspec(dllexport) in Khronos headers
     ])
     if env['gles']:
         env.Append(CPPDEFINES = ['_GLAPI_DLL_EXPORTS'])