Makefile hard coded to use clang

[micolous]

The Makefiles for caption-inspector appear to be all hard coded to use clang:

https://github.com/Comcast/caption-inspector/blob/476326f08a43ce38ecd1ea58b8910d7015e80cac/src/Makefile#L137-L143

This project builds fine on gcc 9.3 too.

One can use the implicit variable $(CC) to get whatever the user-preferred compiler (and its location) is. It also makes it easier to set up a CI build matrix with other compilers.