openGL, GTK and openCV 3.0 supports libwebp.
https://chromium.googlesource.com/webm/libwebp
1. git clone https://chromium.googlesource.com/webm/libwebp
2. build libwebp
./autogen.sh
./configure
make
sudo make install
3. build opencv:
-DWITH_OPENGL:BOOL="1" -DWITH_GTK:BOOL="1" -DWITH_GTK_2_X:BOOL="1" \
-DWITH_WEBP:BOOL="1" -DBUILD_WEBP:BOOL="1" -DBUILD_PNG:BOOL="1" -DWITH_OPENCL:BOOL="1" \