http://download.hexagongeospatial.com/?ProductName=ERDAS%20ECW/JPEG2000%20SDK
Welcome to ERDAS ECW JPEG 2000 SDK 5.2.1 Setup By default setup will install to /Hexagon/EcwJp2SDK5.2.1/$PRODUCT_TYPE depending on the selected product type. To get started installing ERDAS ECW JPEG 2000 SDK 5.2.1 Setup click Continue.
brew install gdal --with-ecw=/Hexagon/ERDASEcwJpeg2000SDK5.2.1/Desktop_Read-Only