I ran into an issue compiling a purple2 plugin on my Debian unstable machine. I
tracked it down to /usr/local/lib/x86_64-linux-gnu being before /usr/local/lib
in my ld.so.conf. This meant that the linker found
/usr/local/lib/x86_64-linux-gnu/libpurple.so.20.0.0 which is purple3, before it
found /usr/local/lib/libpurple.so.14.0.0 which is purple2. This obviously
caused the plugin to be unloadable in purple2/pidgin2.

So to avoid this issue for others, changing the soname just makes sense and is
what other libraries like GTK already do to support running both side by side.

Wiped by build directory then ran the unit tests and pidgin3 and verified everything looked alright.