libsass is only a library and does not do much on its own. You need an implementation that you can use from the command line. Or some bindings|Implementations to use it within your favorite programming language. You should be able to get sassc running by following the instructions in this guide.
Before starting, see setup dev environment.
Building on different Operating Systems
We try to keep the code as OS independent and standard compliant as possible. Reading files from the file-system has some OS depending code, but will ultimately fall back to a posix compatible implementation. We do use some C++11 features, but are so far only committed to use unordered_map. This means you will need a pretty recent compiler on most systems (gcc 4.5 seems to be the minimum).
Building on Linux (and other *nix flavors)
Linux is the main target for libsass and we support two ways to build libsass here. The old plain makefiles should still work on most systems (including MinGW), while the autotools build is preferred if you want to create a system library.
Building on Windows (experimental)
Windows build support was added very recently and should be considered experimental. Credits go to @darrenkopp and @am11 for their work on getting libsass and sassc to compile with visual studio!
Building on Max OS X (untested)
Works the same as on linux, but you can also install LibSass via homebrew.
Building a system library (experimental)
Since libsass is a library, it makes sense to install it as a shared library on your system. On linux this means creating a .so library via autotools. This should work pretty well already, but we are not yet committed to keep the ABI 100% stable. This should be the case once we increase the version number for the library to 1.0.0 or higher. On Windows you should be able get a dll by creating a shared build with MinGW. There is currently no target in the MSVC project files to do this.
Compiling with clang instead of gcc
To use clang you just need to set the appropriate environment variables:
export CC=/usr/bin/clang
export CXX=/usr/bin/clang++
Running the spec test-suite
We constantly and automatically test libsass against the official spec test-suite. To do this we need to have a test-runner (which is written in ruby) and a command-line tool (sassc) to run the tests. Therefore we need to additionally compile sassc. To do this, the build files of all three projects need to work together. This may not have the same quality for all build flavors. You definitely need to have ruby (2.1?) installed (version 1.9 seems to cause problems at least on windows). You also need some gems installed:
ruby -v
gem install minitest
# should be optional
gem install minitap
Including the LibSass version
There is a function in libsass to query the current version. This has to be defined at compile time. We use a C macro for this, which can be defined by calling g++ -DLIBSASS_VERSION="\"x.y.z.\"". The two quotes are necessary, since it needs to end up as a valid C string. Normally you do not need to do anything if you use the makefiles or autotools. They will try to fetch the version via git directly. If you only have the sources without the git repo, you can pass the version as an environment variable to make or configure:
export LIBSASS_VERSION="x.y.z."
Continuous Integration
We use two CI services to automatically test all commits against the latest spec test-suite.
Why not using CMake?
There were some efforts to get libsass to compile with CMake, which should make it easier to create build files for linux and windows. Unfortunately this was not completed. But we are certainly open for PRs!