Fix Signal<std::string>

Add Cmake option to deactivate suffix of so files.

   "[signal-cast-helper] Removed unused template specialization".
   DefaultCastRegisterer<std::string>::cast template specialization is
   required to set empty string to a signal.

  Force tests to link with the library in build directory.

This commit removes template specialization not exposed in header
files, and better handled by the default implementation.

Joseph Mirabel authored 5 years ago and olivier stasse committed 5 years ago

Put class declarations in one and macro definitions in the other.