Project does not build

[mmgen]

System: Linux

I'm following the build instruction in BUILD.md

$ cmake ..
CMake Error at CMakeLists.txt:100 (find_package):
  Could not find a package configuration file provided by "unofficial-sodium"
  with any of the following names:

	unofficial-sodiumConfig.cmake
	unofficial-sodium-config.cmake

  Add the installation prefix of "unofficial-sodium" to CMAKE_PREFIX_PATH or
  set "unofficial-sodium_DIR" to a directory containing one of the above
  files.  If "unofficial-sodium" provides a separate development package or
  SDK, be sure it has been installed.