SONAME support

[HackerSmacker]

Notice

• [ X] I understand the code that I have edited, and have the means
  to test it before making changes to Concord.

What?

This is an exploratory PR response to a request made by Souji Thenria, who requested SONAME support.

Why?

As we make breaking changes, it will become necessary to have a proper SONAME versioning system of sorts.

How?

For Concord 2.3.0, the SONAME is libdiscord.so.MAJOR.MINOR, where MAJOR is 2.3, and MINOR is 1 (in this, I have rolled over the VERSION in src/Makefile to 2.3.1, in anticipation of another release).

Testing?

I tested this on:

• GNU/Linux (GCC, GNU ld)
• FreeBSD (LLVM, LLD)
• Solaris (GCC, GNU ld)

Anything Else?

At some point, it will become necessary to write a new versioning policy, or change that which I have already specified in this PR.

[SoujiThenria]

First of all thank you!

I noticed two issues:

1. When running make uninstall the shared library file is not removed, since the uninstall target only removes libdiscord.so.
2. A symbolic link for libdiscord.so pointing to the ABI versioned file (e.g. libdiscord.so.2.3.1) is missing, causing -ldiscord to not be found when linking against Concord.

[HackerSmacker]

Oops, I legitimately thought I had the ln command in there, I'll fix it in a bit!