sgi: Fix MSVC build.

Including windows.h was ineffective on MSVC because we define the NOGDI macro,
which skips the wingdi.h include.

Unsetting NOGDI is also a bad idea because it causes all sort of symbol
clashes with SGI code.

The real problem is that WINGDAPI was not being defined, also due to NOGDI,
so simply define it to blank if not done already.  This seems to make
everybody happy.

src/include/gluos.h

@@ -60,6 +60,10 @@
 #pragma comment(linker, "/OPT:NOWIN98")
 #endif
 
+#ifndef WINGDIAPI
+#define WINGDIAPI
+#endif
+
 #elif defined(__OS2__)
 
 #include <stdlib.h>

src/libutil/mipmap.c

@@ -6631,7 +6631,7 @@ static TexImage3Dproc pTexImage3D = 0;
 #  include <dlfcn.h>
 #  include <sys/types.h>
 #else
-#  include <windows.h>
+  WINGDIAPI PROC  WINAPI wglGetProcAddress(LPCSTR);
 #endif
 
 static void gluTexImage3D( GLenum target, GLint level,