Examples-Makefile optional compiler assignment

[SoujiThenria]

Notice

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

What?

This pull request changes the assignment of the CC variable to gcc only if CC is not already set to something.

Why?

No other Makefile in this project includes a specific assignment for the CC variable and uses the one defined by the system or the user.
If no gcc is installed on the system, e.g. clang concord will build but the examples not.

How?

The assignment is optional and will be set to gcc if CC is not defined.

Testing?

Testing was done by building Concord and the examples.

[tarbomb]

Why not just cc instead of gcc?

[SoujiThenria]

Why not just cc instead of gcc?

gcc is the current value. I thought there might be a reason for that.
I wouldn't mind changing it to cc.

[tarbomb]

Ah, alright. Yeah, cc is going to be better. This is probably something we should've done from the get-go.

[Ratakor]

Actually this line could be removed because make already set CC by default thus doing CC ?= nul will have no effect too.

[SoujiThenria]

That is true. It is also nowhere set separately besides gencodecs/Makefile. Using the default value set by make would be consistent throughout the project.

[HackerSmacker]

The system-wide make rules are supposed to set CC, but, if /usr/share/mk is nonexistent or broken, boom... yeah, this ought to be changed. the ?= method is clunky!