cmake: fix smpi install dirs (absolute arg)
Previous code assumed that the following variables were relative paths
- CMAKE_INSTALL_INCLUDEDIR
- CMAKE_INSTALL_LIBDIR
However, using absolute paths there is totally legal in CMake,
and doing so broke installed paths in smpicc/smpicxx.
Fix uses the CMAKE_INSTALL_FULL_<dir> variant instead,
which should work in this case and generally be more robust.
FULL variants exist since at least cmake-3.0.0 so it should be available
everywhere (cmake-3.0.0 was released on June 2014).
References:
- https://cmake.org/cmake/help/latest/command/install.html
- https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html
