CMake在编译opencv3.4.0时，需要通过外网下载.cache文件夹中的内容，例如ippicv等

opencv3.3.1 CMake需下载的.cache文件夹 .cache │ .gitignore │ ├─data │ 7505c44ca4eb54b4ab1e4777cb96ac05-face_landmark_model.dat │ ├─dnn_face_detector │ afbb6037fd180e8d2acb3b58ca737b9e-res10_300x300_ssd_iter_140000.caffemodel │ ├─ippicv │ 4e0352ce96473837b1d671ce87f17359-ippicv_2017u3_lnx_intel64_general_20170822.tgz │ ├─tiny_dnn │ adb1c512e09ca2c7a6faef36f9c53e59-v1.0.0a3.tar.gz │ └─xfeatures2d ├─boostdesc │ 0ae0675534aa318d9668f2a179c2a052-boostdesc_lbgm.i │ 0ea90e7a8f3f7876d450e4149c97c74f-boostdesc_bgm.i │ 202e1b3e9fec871b04da31f7f016679f-boostdesc_binboost_064.i │ 232c966b13651bd0e46a1497b0852191-boostdesc_bgm_bi.i │ 324426a24fa56ad9c5b8e3e0b3e5303e-boostdesc_bgm_hd.i │ 98ea99d399965c03d555cef3ea502a0b-boostdesc_binboost_128.i │ e6dcfa9f647779eb1ce446a8d759b6ea-boostdesc_binboost_256.i │ └─vgg 151805e03568c9f490a5e3a872777b75-vgg_generated_120.i 7126a5d9a8884ebca5aea5d63d677225-vgg_generated_64.i 7cd47228edec52b6d82f46511af325c5-vgg_generated_80.i e8d0dcd54d1bcfdc29203d011a797179-vgg_generated_48.i9a8884ebca5aea5d63d677225-vgg_generated_64.i 7cd47228edec52b6d82f46511af325c5-vgg_generated_80.i e8d0dcd54d1bcfdc29203d011a797179-vgg_generated_48.i