The source code is available from a git repository hosted at Sourceforge. You can check out the current head revision by

    >$ git clone lensfun-code

The build system is based on CMake. Enter the Lensfun folder and create the CMake build directory:

    >$ cd lensfun-code/
    >$ mkdir build
    >$ cd build/
    >$ cmake ../

Beneath many other files this should have created a Makefile that can now be used to compile and install as usual with

    >$ make
    >$ make install

More information about the build system and available options can be found in the README file in the root folder.