Fix dedicated server for Windows

* the OpenAL function definitions mustn't include __declspec(dllimport)
  => fixed by pretending to compile OpenAL statically
* glimp.cpp shouldn't be used in dedicated-only mode (as it was already
  the case on Linux and OSX)
  => No special handling for ID_DEDICATED needed in glimp.cpp, as it's not
   used anyway
* add APIENTRY to every gl function in stub_gl.cpp for compatibility
  with windows headers and MSVC
* remove GL/gl.h #include from win_local.h as it's not needed
* in qgl.h, when building dedicated server for windows, redefine WINGDIAPI
  to nothing for SDL_opengl.h #include to get rid off __declspec(dllimport)
  by using #pragma push_macro and pop_macro, because our stub is no dll.

Fixes https://github.com/dhewm/dhewm3/issues/39

CMakeLists.txt

@@ -693,7 +693,6 @@ elseif(WIN32)
 	set(src_sys_base
 		sys/cpu.cpp
 		sys/threads.cpp
-		sys/glimp.cpp
 		sys/events.cpp
 		sys/sys_local.cpp
 		sys/win32/win_input.cpp
@@ -705,6 +704,7 @@ elseif(WIN32)
 	)
 
 	set(src_sys_core
+		sys/glimp.cpp
 	)
 else()
 	set(src_sys_base

renderer/qgl.h

@@ -32,8 +32,25 @@ If you have questions concerning this license or the applicable additional terms
 #ifndef __QGL_H__
 #define __QGL_H__
 
+#if defined( ID_DEDICATED ) && defined( _WIN32 )
+// to allow stubbing gl on windows, define WINGDIAPI to nothing - it would otherwise be
+// extended to __declspec(dllimport) on MSVC (our stub is no dll.)
+	#ifdef WINGDIAPI
+		#pragma push_macro("WINGDIAPI")
+		#undef WINGDIAPI
+		#define WINGDIAPI
+	#endif
+#endif
+
 #include <SDL_opengl.h>
 
+#if defined( ID_DEDICATED ) && defined( _WIN32 )
+// restore WINGDIAPI
+	#ifdef WINGDIAPI
+		#pragma pop_macro("WINGDIAPI")
+	#endif
+#endif
+
 typedef void (*GLExtension_t)(void);
 
 #ifdef __cplusplus