build glow as static library on OSX

load-time resolution of twice-statically-linked imgui symbols fails
when glow-extras is linked as a shared library.

CMakeLists.txt

@@ -40,8 +40,9 @@ if (GLOW_CMAKE_REPORT)
     message(STATUS "  OpenGL           : ${GLOW_OPENGL_SUPPORT}")
 endif()
 
-if(MSVC)
-    set(GLOW_LINK_TYPE STATIC CACHE STRING "" FORCE) # unfortunately, we need to link statically here
+if(MSVC OR APPLE)
+    # we need to link statically on Windows and OSX
+    set(GLOW_LINK_TYPE STATIC CACHE STRING "" FORCE)
 endif()
 
 if(APPLE)