Reactivated the UIUC, LaRCsim, and special purpose FDMs.

Reactivated the UIUC, LaRCsim, and special purpose FDMs.

As the FlightGear distribution ships aircraft based on these FDMs with each
release, the shipped FlightGear executables should therefore be able to handle
these aircraft.  For example the unique ornithopter can only be flown with the
UIUC FDM, hence it is available for download but will be non-functional for most
users.

This partly reverts commit e819a4aaa44da6be816460fe719b1f65e0a2db3c from Nov 1,
2011 which deactivated the compilation of these in the CMakeLists.txt file.

Signed-off-by: Edward d'Auvergne <edward@nmr-relax.com>