#179 Add interface to select arch specific optimizations

[Gert Wollny]

It should be possible to provide several plug-ins of the same name that contain arch specific optimizations that are selected at run-time based on the available CPU.

[Gert Wollny]

Currently being implemented in the branch 'plugin-priority' is a method to assign a priority to a plug-in. If a second plug-in with the same name is loaded that has a higher priority, then it replaces the first one.
A plug-in with higher priority implementing the same function will only provide the interface if the optimizations are actually supported on the architecture used to run the code.

The major problem is that one has to avoid name clashes of static symbols. This can either be done with namespaces or (hopefully) by unloading the unused plug-in(s).

[Gert Wollny]

The latter problem can be alleviated by compiling everything with

-fvisibility=hidden

which will be forced from the root CMakeLists.txt file in the future.