libsocialweb: conditional compilation
libsocialweb: conditional compilation

Introduces --enable-libsocialweb-backend=[yes/no/auto] (default: auto). Unless
`no' is used, it checks the dependencies for the libsocialweb backend and
automatically compiles it when possible. Configure fails if `yes' is used and
the required dependencies are not available.

renatofilho/folks,renatofilho/folks,renatofilho/folks2,renatofilho/folks2,renatofilho/folks,pwithnall/folks,renatofilho/folks2,GNOME/folks,pwithnall/folks,GNOME/folks,pwithnall/folks,renatofilho/folks2,GNOME/folks,renatofilho/folks