[DFT] Make options for building DFT and its features

This patch adds the following three CMAKE build options.

* BUILD_DFT : Turned on by default. libsleefdft will be built if turned on.
* SLEEFDFT_ENABLE_LONGDOUBLE : Turned off by default. Long double DFT subroutines are compiled in if turned on.
* SLEEFDFT_ENABLE_QUAD : Turned off by default. Quad precision DFT subroutines are compiled in if turned on.