Alien and static libraries

[cedricchevalier19]

Currently multi-representation needs back-ends to register themself. This is done in the constructor of a static object. This object is not referenced anywhere in Alien's code, so static linking can work only using --whole-archive.

It should work with shared libraries, however on ubuntu, one may need to add this option CMAKE_EXE_LINKER_FLAGS="-Wl,--no-as-needed" to avoid dismissing non referenced libraries.