apply tgsm's changes (see PR #411 )

src/CMakeLists.txt

@@ -28,7 +28,7 @@ add_library(core STATIC
 )
 
 if (WIN32)
-	target_link_libraries(core ole32 comctl32 ws2_32)
+	target_link_libraries(core ole32 comctl32 ws2_32 opengl32)
 else()
 	target_link_libraries(core OpenGL)
 endif()

src/libui_sdl/libui/windows/CMakeLists.txt

@@ -29,6 +29,7 @@ list(APPEND _LIBUI_SOURCES
 	windows/fontbutton.cpp
 	windows/fontdialog.cpp
 	windows/form.cpp
+	windows/gl.cpp
 	windows/graphemes.cpp
 	windows/grid.cpp
 	windows/group.cpp