Fix CMake Bug: ensure cmake_install_prefix is set.
Setting CMAKE_INSTALL_PREFIX only seems to have an effect if it happens before the call to project() (see https://cmake.org/pipermail/cmake/2010-December/041133.html).
Setting CMAKE_INSTALL_PREFIX only seems to have an effect if it happens before the call to project() (see https://cmake.org/pipermail/cmake/2010-December/041133.html).