Added the bundled clFFT into OpenCL builds
Used an object library, since we have no need of a real library, to
have or to install, whether shared or static. Checked for the
availability of dynamic loading, and made it available portably to
libgromacs.
Clfft initialization class is added and used in mdrunner to
initialize/tear down clFFT library resources in a thread-safe
manner, and only on ranks that require such setup. Noted TODOs
for future work.
Noted a useful style for explicit listing of source files.
Refs #2500
Refs #2515
Refs #2535
Change-Id: I62d7d66f65e147bde17929ccc30abad36e2373c6