You can get the celerite source code by running
git clone https://github.com/dfm/celerite.git
celerite is a header-only library and the header files can be found in the cpp/include
directory in the source tree.
The only hard dependency is a recent version of Eigen. celerite has been tested with Eigen 3.2.9 but somewhat older versions should also work.
The unit tests can be run using CMake. Navigate to the cpp
directory and execute:
cmake .
make
make test