Don't build CG on Linux CMake

cmake/CMakeLists.txt

 # Boilerplate.
 cmake_minimum_required (VERSION 3.1)   # First version with CMAKE_CXX_STANDARD.
 project (skimake)
 set (CMAKE_CXX_STANDARD 11)
 
 # Default to Release mode.  We're mainly targeting Skia users, not Skia developers.
 if (NOT CMAKE_BUILD_TYPE)
     set (CMAKE_BUILD_TYPE Release)
 endif ()
 
@@ skipping 87 matching lines @@
     ../src/gpu/vk/*
     ../src/opts/SkBitmapProcState_opts_none.cpp
     ../src/opts/SkBlitMask_opts_none.cpp
     ../src/opts/SkBlitRow_opts_none.cpp
     ../src/ports/SkFontMgr_empty_factory.cpp
     ../src/ports/SkGlobalInitialization_chromium.cpp
     ../src/ports/SkImageDecoder_empty.cpp
     ../src/ports/SkImageGenerator_none.cpp
     ../src/ports/SkTLS_none.cpp)
 
+if (NOT APPLE)
+    remove_srcs(../src/ports/SkImageGeneratorCG.cpp)
+endif()
+
 if (WIN32)
     if(SKIA_GDI)
         remove_srcs(../src/ports/SkFontMgr_win_dw_factory.cpp)
     else()
         remove_srcs(../src/ports/SkFontMgr_win_gdi_factory.cpp)
     endif()
 endif()
 
 remove_srcs(../src/gpu/gl/angle/*)  # TODO
 
@@ skipping 204 matching lines @@
         file (APPEND ${skia_h_path} "#include \"${filename_component}\"\n")
       endif ()
     endforeach()
   endif()
 endforeach()
 file(APPEND ${skia_h_path} "\n#endif  // skia_DEFINED\n")
 
 # Now build a simple example app that uses Skia via libskia.so.
 add_executable(example example.cpp)
 target_link_libraries(example skia ${OPENGL_LIBRARIES})