configure: minor update to the inline help
hard-code math constants as literals

Clang doesn't cope with constexpr of function type (pi=acos(-1),
sqrt2=sqrt(2)).
Fix bucketload of warnings raised by clang

Tested using clang++ 3.5.0 on Arch Linux - hopefully these changes will
work out of the box on MacOSX too.

Also slight change in get_func()/put_func() handling of Image::Buffer,
and simpler handling of Image::ThreadedLoop::run()

Everything now compiles clean under clang, apart from:

    - many of the __get() / __put() function specialisations in
      Image::Buffer raise warnings due to being unused.

    - warnings are raised for src/gui/mrview/image.cpp due to a needless
      call to std::abs in template specialisations for unsigned types.
      Could be fixed using SFINAE approaches, but it's just not worth
      the effort.

Also re-enable full warnings for the compiler.
Image::Buffer: reworked __get()/__put() methods

This was mostly motivated by the huge number of unused function warnings
issued by clang. This is a much cleaner C++11 implementation, with some
very fine SFINAE magic, IMNSHO...

Tested using clang 3.5.0 & GCC 4.9.2 on Arch Linux.