The number convention of the library webp is confusing. Because, when you install "libwebp-0.5.0" source package from Google developers website; it will install libwebp.6 in your mac lib folders.
To solve this kindly go to the repositories location from Google developers webiste and download the "libwebp-0.4.4"source package.
Go to the directory where
libwebp-0.4.4/was extracted to and run the following commands:
sudo make install
This should solve the problem