CMake download issue when building OpenCV behind proxy - opencv

Question

I'm trying to install OpenCV 3.1.0 (with contrib) on CentOS 7 behind a proxy. I get download errors for the 3rd party libraries. I've tried setting the HTTP_PROXY (also http_proxy, https_proxy, ftp_proxy and their uppercase versions) environment variable but it doesn't make a difference. Here's the command I use:
cmake -D CMAKE_BUILD_TYPE=RELEASE \
-D CMAKE_INSTALL_PREFIX=/usr/local \
-D OPENCV_EXTRA_MODULES_PATH=~/opencv_contrib/modules \
-D INSTALL_C_EXAMPLES=OFF \
-D INSTALL_PYTHON_EXAMPLES=ON \
-D BUILD_EXAMPLES=ON \
-D BUILD_OPENCV_PYTHON2=ON ..
And here's the error I get:
-- Detected version of GNU GCC: 48 (408)
-- Found ZLIB: /lib64/libz.so (found suitable version "1.2.7", minimum required is "1.2.3")
-- Found ZLIB: /lib64/libz.so (found version "1.2.7")
-- Found OpenEXR: /lib64/libIlmImf.so
-- checking for module 'gtk+-3.0'
-- package 'gtk+-3.0' not found
-- checking for module 'gstreamer-base-1.0'
-- package 'gstreamer-base-1.0' not found
-- checking for module 'gstreamer-video-1.0'
-- package 'gstreamer-video-1.0' not found
-- checking for module 'gstreamer-app-1.0'
-- package 'gstreamer-app-1.0' not found
-- checking for module 'gstreamer-riff-1.0'
-- package 'gstreamer-riff-1.0' not found
-- checking for module 'gstreamer-pbutils-1.0'
-- package 'gstreamer-pbutils-1.0' not found
-- Looking for linux/videodev.h
-- Looking for linux/videodev.h - not found
-- Looking for linux/videodev2.h
-- Looking for linux/videodev2.h - found
-- Looking for sys/videoio.h
-- Looking for sys/videoio.h - not found
-- checking for module 'libavcodec'
-- package 'libavcodec' not found
-- checking for module 'libavformat'
-- package 'libavformat' not found
-- checking for module 'libavutil'
-- package 'libavutil' not found
-- checking for module 'libswscale'
-- package 'libswscale' not found
-- checking for module 'libavresample'
-- package 'libavresample' not found
-- Looking for libavformat/avformat.h
-- Looking for libavformat/avformat.h - not found
-- Looking for ffmpeg/avformat.h
-- Looking for ffmpeg/avformat.h - not found
-- checking for module 'libgphoto2'
-- package 'libgphoto2' not found
CMake Warning at 3rdparty/ippicv/downloader.cmake:56 (message):
ICV: Local copy of ICV package has invalid MD5 hash:
d41d8cd98f00b204e9800998ecf8427e (expected:
808b791a6eac9ed78d32a7666804320e)
Call Stack (most recent call first):
3rdparty/ippicv/downloader.cmake:110 (_icv_downloader)
cmake/OpenCVFindIPP.cmake:237 (include)
cmake/OpenCVFindLibsPerf.cmake:12 (include)
CMakeLists.txt:537 (include)
-- ICV: Downloading ippicv_linux_20151201.tgz...
CMake Error at 3rdparty/ippicv/downloader.cmake:73 (file):
file DOWNLOAD HASH mismatch
for file: [/home/hal/opencv/3rdparty/ippicv/downloads/linux-808b791a6eac9ed78d32a7666804320e/ippicv_linux_20151201.tgz]
expected hash: [808b791a6eac9ed78d32a7666804320e]
actual hash: [d41d8cd98f00b204e9800998ecf8427e]
Call Stack (most recent call first):
3rdparty/ippicv/downloader.cmake:110 (_icv_downloader)
cmake/OpenCVFindIPP.cmake:237 (include)
cmake/OpenCVFindLibsPerf.cmake:12 (include)
CMakeLists.txt:537 (include)
CMake Error at 3rdparty/ippicv/downloader.cmake:77 (message):
ICV: Failed to download ICV package: ippicv_linux_20151201.tgz.
Status=7;"Couldn't connect to server"
Call Stack (most recent call first):
3rdparty/ippicv/downloader.cmake:110 (_icv_downloader)
cmake/OpenCVFindIPP.cmake:237 (include)
cmake/OpenCVFindLibsPerf.cmake:12 (include)
CMakeLists.txt:537 (include)
-- Configuring incomplete, errors occurred!