To install LibSass, make sure the OS X build tools are installed:
xcode-select --install
To install homebrew, see http://brew.sh
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
You can install the latest version of LibSass quite easily with brew.
brew install --HEAD libsass
To update this, do:
brew reinstall --HEAD libsass
Brew will build static and shared libraries, and a libsass.pc file in /usr/local/lib/pkgconfig.
To use libsass.pc, make sure this path is in your PKG_CONFIG_PATH
export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig
See the linux instructions Building-with-autotools or Building-with-makefiles