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Also ensure FFTW variables only defined if FFTW is required and to ensure the
header is correctly found, rather than assuming it is in an include directory
at the same level as the library directory (not true with multi-arch).

Switch to a simple static member function to implement singletons, which is guaranteed to be thread-safe in C++11.

XDMF ships with a subset of Loki files on Debian, which does not include the
Singleton.h header we need.