Change the soname of libpurple to libpurple3

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.

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

Reviewed at https://reviews.imfreedom.org/r/2182/